Online Etymology Dictionary

th: A sound found chiefly in words of Old English, Old Norse or Greek origin, unpronounceable by Normans and many other Europeans. In Greek, the sound corresponds etymologically to Sanskrit -dh- and English -d-; and it was represented graphically by -TH- and at first pronounced as a true aspirate (as still in English outhouse, shithead, etc.). But by 2c. B.C.E. the Greek letter theta was in universal use and had the modern "-th-" sound. Latin had neither the letter nor the sound, however, and the Romans represented Greek theta by -TH-, which they generally pronounced, at least in Late Latin, as simple "-t-" (passed down to Romanic languages, as in Spanish termal "thermal," teoria "theory," teatro "theater").

In Germanic languages it represents PIE *-t- and was common at the start of words or after stressed vowels. To represent it, Old English and Old Norse used the characters ð "eth" (a modified form of -d-) and þ "thorn," which originally was a rune. Old English, unlike Old Norse, seems never to have standardized which of the two versions of the sound ("hard" and "soft") was represented by which of the two letters.

The digraph -th- sometimes appears in early Old English, on the Roman model, and it returned in Middle English with the French scribes, driving out eth by c. 1250, but thorn persisted, especially in demonstratives (þat, þe, þis, etc.), even as other words were being spelled with -th-. The advent of printing dealt its death-blow, however, as types were imported from continental founders, who had no thorn. For a time y was used in its place (especially in Scotland), because it had a similar shape, hence ye for the in historical tourist trap Ye Olde _______ Shoppe (it never was pronounced "ye," only spelled that way).

The awareness that some Latin words in t- were from Greek th- encouraged over-correction in English and created unetymological forms such as Thames and author, while some words borrowed from Romanic languages preserve, on the Roman model, the Greek -th- spelling but the simple Latin "t" pronunciation (as in Thomas and thyme).
-th (2): suffix forming nouns of action, state, or quality from verbs or adjectives (such as depth, strength, truth), from Old English -ðu, -ð, from Proto-Germanic *-itho (cognates: Old Norse -þ, Old High German -ida, Gothic -iþa), abstract noun suffix, from PIE *-ita (cognates: Sanskrit -tati-; Greek -tet-; Latin -tati-, as in libertatem "liberty" from liber "free"). Sometimes in English reduced to -t, especially after -h- (as in height).
-th (1): word-forming element making ordinal numbers (fourth, tenth, etc.), Old English -ða, from Proto-Germanic *-tha- (cognates: Gothic -da, -ta, Old High German -do, -to, Old Norse -di, -ti), from PIE *-to-, also *-eto-, *-oto-, suffix forming adjectives "marking the accomplishment of the notion of the base" [Watkins]. Cognate with Sanskrit thah, Greek -tos, Latin -tus; Sanskrit ta-, Lithuanian and Old Church Slavonic to, Greek to "the," Latin talis "such;" Greek telikos "so old," Old Church Slavonic toli "so," toliku "so much," Russian toliko "only;" also see -ed.
ye (article): old or quaintly archaic way of writing the, in which the -y- is a 16c. graphic alteration of þ, an Old English character (generally called "thorn," originally a Germanic rune; see th-) that represented the -th- sound (as at the beginning of thorn). The characters for -y- and -þ- so closely resembled each other in Old English and early Middle English handwriting that a dot had to be added to the -y- to keep them distinct. In late 15c., early printers in English, whose types were founded on the continent, did not have a þ in their sets, so they substituted y as the letter that looked most like it when setting type. But in such usages it was not meant to be pronounced with any of the sounds associated with -y-, but still as "-th-." Ye for the (and yt for that) continued in manuscripts through 18c. Revived 19c. as a deliberate antiquarianism; the Ye Olde _____ construction was being mocked by 1896.
futhorc (n.): 1851, historians' name for the Germanic runic alphabet; so called from its first six letters (th being a single rune), on the model of alphabet.
fifteenth (adj.): late 14c., from fifteen + -th (1). By 15c. displacing forms derived from Old English fifteoða. Compare Old Frisian fiftuda, Dutch vijftiende, German fünfzehnte, Old Norse fimmtandi, Gothic fimftataihunda, with ordinal -d where English has -th. As a noun by late 14c.
Thames: river through London, Old English Temese, from Latin Tamesis (51 B.C.E.), from British Tamesa, an ancient Celtic river name perhaps meaning "the dark one." The -h- is unetymological (see th).
theta (n.): eighth letter of the Greek alphabet; in ancient Greece, from Hebrew teth; originally an aspirated -t- (see th). Written on ballots to indicate a vote for a sentence of "death" (thanatos), hence occasional allusive use for "death."
tandem (n.): 1785, "carriage pulled by horses harnessed one behind the other" (instead of side-by-side), jocular use of Latin tandem "at length (of time), at last, so much," from tam "so" (from PIE *tam-, adverbial form of demonstrative pronoun root *-to-; see -th (1)) + demonstrative suffix -dem. "Probably first in university use" [Century Dictionary]. Transferred by 1884 to bicycles with two seats. In English as an adverb from 1795; as an adjective from 1801.
drought (n.): Old English drugað, drugoð "drought, dryness, desert," from Proto-Germanic *drugothaz, from Germanic root *dreug- "dry" (cf high/height) with *-itho, Germanic suffix for forming abstract nouns (see -th (2)). Drouth was a Middle English variant continued in Scottish and northern English dialect and in poetry.
health (n.): Old English hælþ "wholeness, a being whole, sound or well," from Proto-Germanic *hailitho, from PIE *kailo- "whole, uninjured, of good omen" (source also of Old English hal "hale, whole;" Old Norse heill "healthy;" Old English halig, Old Norse helge "holy, sacred;" Old English hælan "to heal"). With Proto-Germanic abstract noun suffix *-itho (see -th (2)). Of physical health in Middle English, but also "prosperity, happiness, welfare; preservation, safety." An abstract noun to whole, not to heal. Meaning "a salutation" (in a toast, etc.) wishing one welfare or prosperity is from 1590s. Health food is from 1848.
billionth: 1778, from billion + -th (2).
millionth (adj.): 1670s, from million + -th (1).
thousandth (adj.): 1550s, from thousand + -th (1).
trillionth: 1820, from trillion + -th (1).
seventieth (adj.): late 13c., from seventy + -th (1).
eth (n.): name of an Anglo-Saxon runic character (Ð, ð) representing the sound "-th-," 1846, from th + e, "the usual assistant vowel in letter-names" [Century Dictionary].
thorn (n.): Old English þorn "sharp point on a stem or branch," earlier "thorny tree or plant," from Proto-Germanic *thurnuz (source also of Old Saxon, Old Frisian thorn, Dutch doorn, Old High German dorn, German Dorn, Old Norse þorn, Gothic þaurnus), from PIE *trnus (source also of Old Church Slavonic trunu "thorn," Sanskrit trnam "blade of grass," Greek ternax "stalk of the cactus," Irish trainin "blade of grass"), from *(s)ter-n- "thorny plant," from root *ster- (1) "stiff" (see stark).

Figurative sense of "anything which causes pain" is recorded from early 13c. (thorn in the flesh is from II Cor. xii:7). Also an Anglo-Saxon and Icelandic runic letter (þ), named for the word of which it was the initial (see -th-).
sixtieth: Old English sixteogoða "sixtieth;" see sixty + -th (1).
thirtieth: Middle English threttyth, from Old English þritigoða; see thirty + -th (1). Respelled 16c. to conform to new spelling of thirty.
ninetieth (adj.): late 14c., nyntithe, from ninety + -th (1); a replacement or modification of Old English nigenteoþan.
seventeenth (adj.): c. 1300, seventenþe, from seventeen + -th (1); replacing forms based on Old English seofonteoþa.
spilth (n.): "that which is spilled," c. 1600, from spill (v.) + -th (2). Used, once, by Shakespeare.
tilth (n.): "labor, work" (especially in agriculture), Old English tilþ "labor, husbandry," from tilian "to till" (see till (v.)) + -th (2).
fortieth (adj.): c. 1200, fowertiþe, from Old English feowertigoða, from feowertig (see forty) + -th (1). Compare Old Norse fertugonde, Swedish fyrationde, Danish fyrretyvende.
warmth (n.): late 12c., wearmth, Proto-Germanic *warmitho- (source also of Middle Low German wermede, Dutch warmte), from *warmo- (see warm (adj.); also see -th (2)).
fiftieth (adj.): Old English fifteogoða "fiftieth;" see fifty + -th (1). Compare Old Norse fimmtugande, and, with a different suffix, Old Frisian fiftichsta, Dutch vijftigste, Old High German fimfzugsto, German fünfzigste.
fourteenth (adj.): c. 1300, fourtenethe; see fourteen + -th (1). By influence of fourteen, replacing or modifying fourtende, fowrtethe, from Old English feowerteoða, Old Norse *feowertandi "fourteenth." Compare Dutch veertiende, German vierzehnte.
eighth: late 14c., eighthe, contracted from Old English eahtoða; see eight + -th (1). Cognate with Old High German ahtoda, Old Frisian achta, German achte, Gothic ahtuda.
nineteenth (adj.): late 14c., nyntenthe; from nineteen + -th (1); a replacement or modification of nigonteoþa, from Old English nigon-teoða. Nineteenth hole "bar-room in a golf clubhouse" is attested from 1901.
twentieth (adj.): 16c., from twenty + -th (1), replacing Middle English twentithe, from Old English twentigoða. The Twentieth Century Limited was an express train from New York to Chicago 1902-1967.
-teenth: word-forming element making ordinal numbers from 13 to 19, from -teen + -th (1), displacing Old English -teoða, -teoðe (West Saxon), related to teogoða (Anglian) "tenth."
theft (n.): mid-13c., from Old English þeofð (West Saxon þiefð) "theft," from Proto-Germanic *theubitho (source also of Old Frisian thiufthe, Old Norse þyfð), from *theubaz "thief" (see thief) + abstract formative suffix *-itha (cognate with Latin -itatem; see -th (2)).
mirth (n.): Old English myrgð "joy, pleasure," from Proto-Germanic *murgitha (source also of Middle Dutch merchte), noun of quality from *murgjo- (see merry; also see -th (2)). Mirthquake "entertainment that excites convulsive laughter" first attested 1928, in reference to Harold Lloyd movies.
seventh: c. 1300, from seven + -th (1); earlier sevende, seveth, from Old English seofunda (Anglian, Northumbrian), seofoþa (West Saxon), from Proto-Germanic *sebundon (source also of Old Norse sjaundi, Old Saxon sivondo, Old High German sibunto, German siebente, siebte). The music note sense is from 1590s.
hundredth (n.): early 14c., "one of 100 equal parts," from hundred + -th (1). According to OED, Old English had no ordinal form. The hymn tune known as Old Hundredth is attested from mid-16c., so called because later it was set to the 100th Psalm in the old numbering of the psalter.
growth (n.): 1550s, "stage in growing," from grow + -th (2), on model of health, stealth, etc. Compare Old Norse groði, from groa "to grow." Meaning "that which has grown" is from 1570s; "process of growing" is from 1580s. Old English used grownes "increase, prosperity."
sixteenth (adj.): early 13c., from sixteen + -th (1); replacing sixtethe, sixteothe, forms based on Old English syxteoða. Cf Old Frisian sextinda, Middle Dutch sestiende, German sechzehnte, Old Norse sextandi. Musical sixteenth note is from 1861.
dearth (n.): mid-13c., derthe "scarcity" (originally used of famines, when food was costly because scarce; extended to other situations of scarcity from early 14c.), abstract noun formed from root of Old English deore "precious, costly" (see dear) + abstract noun suffix -th (2). Common Germanic formation, though not always with the same sense (cognates: Old Saxon diurtha "splendor, glory, love," Middle Dutch dierte, Dutch duurte, Old High German tiurida "glory").
depth (n.): late 14c., apparently formed in Middle English on model of length, breadth; from Old English deop "deep" (see deep) + -th (2). Replaced older deopnes "deepness." Though the English word is relatively recent, the formation is in Proto-Germanic, *deupitho-, and corresponds to Old Saxon diupitha, Dutch diepte, Old Norse dypð, Gothic diupiþa.
berth (n.): 1620s, "convenient sea room" (both for ships and sailors), of uncertain origin, probably from bear (v.) + abstract noun suffix -th (2) as in strength, health, etc. Original sense is preserved in phrase to give (something or someone) wide berth. Meaning "place on a ship to stow chests, room for sailors" is from 1706; extended to non-nautical situations by 1778.
twelfth (adj.): late 14c., with -th (1), altering Middle English twelfte, from Old English twelfta, from twelf ((see twelve). The earlier form is cognate with Old Norse tolfti, Danish tolvte, Old Frisian twelefta, Dutch twaalfde, Old High German zwelifto, German zwölfte .

As a noun meaning "a twelfth part," from 1550s. Twelfth Night is Old English twelftan niht "Twelfth Night," the eve of Epiphany, which comes twelve days after Christmas, formerly an occasion of social rites and a time of merrymaking.
sloth (n.): late 12c., "indolence, sluggishness," formed from Middle English slou, slowe (see slow (adj.)) + abstract formative -th (2). Replaced Old English slæwð "sloth, indolence." Sense of "slowness, tardiness" is from mid-14c. As one of the deadly sins, it translates Latin accidia.

The slow-moving mammal first so called 1610s, a translation of Portuguese preguiça "slowness, slothfulness," from Latin pigritia "laziness" (compare Spanish perezosa "slothful," also "the sloth").
eleventh (adj.): late 14c., eleventhe, superseding earlier ellefte (c. 1300), enlefte (early 13c.), from Old English endleofta; see eleven + -th (1). Eleventh hour "last moment, just before it is too late" is in Old English, from the parable of the laborers in the vineyard (Matt. xx:1-16); as an adjective by 1829.
thirteenth (adj.): 15c. metathesis of þriteenþe (mid-14c.; see thirteen + -th (1)), which replaced forms from Old English þreoteoða (West Saxon), þriteogeða (Anglian). Similar formation in Old Norse þrettande, Danish trettende, Swedish trettonde, Old Frisian threttinde, Dutch dertiende, Old High German dritto-zehanto, German dreizehnte.
stealth (n.): mid-13c., "theft, action or practice of stealing," from a probable Old English *stælþ, which is related to stelen (see steal (v.)), from Proto-Germanic *stælitho (source also of Old Norse stulþr), with Proto-Germanic abstract noun suffix *-itho (see -th (2)).

Compare heal/health, weal/wealth. Sense of "secret action" developed c. 1300, but the word also retained its etymological sense into 18c. Got a boost as an adjective from stealth fighter, stealth bomber, radar-evading U.S. military aircraft, activated 1983.
several (adj.): early 15c., "existing apart," from Anglo-French several, from Middle French seperalis "separate," from Medieval Latin separalis, from Latin separ "separate, different," back-formation from separare "to separate" (see separate (v.)). Meaning "various, diverse, different" is attested from c. 1500; that of "more than one" is from 1530s, originally in legal use.
Here we are all, by day; by night we're hurled
By dreams, each one into a several world
[Herrick, 1648]
Related: Severalty. Jocular ordinal form severalth attested from 1902 in American English dialect (see -th (2)).
height (n.): Old English hiehþu, Anglian hehþo "highest part or point, summit; the heavens, heaven," from root of heah "high" (see high) + -itha, Germanic abstract noun suffix (as in width, depth; see -th (2)). Compare Old Norse hæð, Middle Dutch hoochte, Old High German hohida, Gothic hauhiþa "height." Meaning "distance from bottom to top" is from late 13c. Meaning "excellence, high degree of a quality" is late 14c. Century Dictionary says "there is no reason for the distinction of vowel between high and height. The modern pronunciation with -t emerged 13c. but wasn't established until 19c.; Milton used highth and heighth is still colloquial in English. Compare Dutch hoogte, Danish hjöde.
sixth (adj.): 1520s, replacing Middle English sixte (c. 1200), from Old English syxte, from siex (see six). Compare Old Frisian sexta, Middle Dutch seste, Old High German sehsto, German sechste, Gothic saihsta. With ending conformed to -th (1). Related: Sixthly. The noun meaning "a sixth part" is from 1550s. As a music tone, from 1590s. Sixth sense "supernatural perception of objects" is attested from 1712; earlier it meant "titillation, the sense that apprehends sexual pleasure" (1690s, from Scaliger).
Then said Peter, That is false; for there is a sixth Sense, that of Prescience : for the other five Senses are capable only of Knowledg ; but the Sixth of Foreknowledg ; which Sense the Prophets had. [William Whitson, "Primitive Christianity Reviv'd," vol. V, London, 1712]
length (n.): Old English lengðu "property of being long or extended in one direction; distance along a line," from Proto-Germanic *langitho, abstract noun from *langaz "long" (root of Old English lang; see long (adj.)) + *-itho, abstract noun suffix (see -th (2)). Cognate with Old Norse lengd, Old Frisian lengethe, Dutch lengte.

Figurative sense of "the distance one goes, extremity to which something is carried" is from 1690s. Phrase at length "to full extent" is attested from c. 1500. As "the length of a swimming pool," 1903. From the notion of "a piece or portion of the extent of anything" come the theater slang sense "a 42-line portion of an actor's part" (1736) and the sporting sense "the length of a horse, car, etc. in a race" used as a unit of measure (1650s).