Online Etymology Dictionary

think (v.): Old English þencan "imagine, conceive in the mind; consider, meditate, remember; intend, wish, desire" (past tense þohte, past participle geþoht), probably originally "cause to appear to oneself," from Proto-Germanic *thankjan (source also of Old Frisian thinka, Old Saxon thenkian, Old High German denchen, German denken, Old Norse þekkja, Gothic þagkjan).

Old English þencan is the causative form of the distinct Old English verb þyncan "to seem, to appear" (past tense þuhte, past participle geþuht), from Proto-Germanic *thunkjan (source also of German dünken, däuchte). Both are from PIE *tong- "to think, feel" which also is the root of thought and thank.

The two Old English words converged in Middle English and þyncan "to seem" was absorbed, except for its preservation in archaic methinks "it seems to me." As a noun, "act of prolonged thinking," from 1834. The figurative thinking cap is attested from 1839.
think tank (n.): also think-tank, 1959 as "research institute" (first reference is to Center for Behavioral Sciences, Palo Alto, Calif.); it had been colloquial for "the brain" since 1905. See think + tank (n.).
thinker (n.): "one who has cultivated the powers of thought," mid-15c., agent noun from think (v.).
thinkable (adj.): 1805; see think (v.) + -able. Possibly a back-formation from unthinkable.
free-thinker (n.): "one not guided in belief by authority; one who submits the claims of authority to what he deems the test of reason," 1690s, from free (adj.) + think (v.) + agent noun suffix -er (1). Free-thought "rationalism" is from 1711. Related: Free-thinking.
thank (v.): Old English þancian, þoncian "to give thanks, thank, to recompense, reward," from Proto-Germanic *thankojan (source also of Old Saxon thancon, Old Norse þakka, Danish takke, Old Frisian thankia, Old High German danchon, Middle Dutch, Dutch, German danken "to thank"), from *thankoz "thought, gratitude," from PIE root *tong- "to think, feel."

Related phonetically to think as song is to sing; for sense evolution, compare Old High German minna "loving memory," originally "memory." Related to Old English noun þanc, þonc, originally "thought," but by c. 1000 "good thoughts, gratitude." In ironical use, "to blame," from 1550s. To thank (someone) for nothing is recorded from 1703. Related: Thanked; thanking.
thought (n.): Old English þoht, geþoht "process of thinking, a thought; compassion," from stem of þencan "to conceive of in the mind, consider" (see think). Cognate with the second element in German Gedächtnis "memory," Andacht "attention, devotion," Bedacht "consideration, deliberation." Second thought "later consideration" is recorded from 1640s.

Bammesberger ("English Etymology") explains that in Germanic -kt- generally shifted to -ht-, and a nasal before -ht- was lost. Proto-Germanic *thankija- added a suffix -t in the past tense. By the first pattern the Germanic form was *thanht-, by the second the Old English was þoht.

Thought-crime is from "Nineteen Eighty-Four" (1949); thought police is attested from 1945, originally in reference to war-time Japanese Special Higher Police (Tokubetsu Koto Keisatsu).
groupthink (n.): 1959, from group (n.) + think.
bethink (v.): reflexive verb, Old English beþencan "to consider," from be- + þencan "to think" (see think). Related: Bethought.
rethink (v.): 1700, from re- "back, again" + think (v.). Related: Rethinking.
misthink (v.): Old English misðyncan "to be mistaken;" see mis- (1) + think (v.). From early 13c. as "to have sinful thoughts;" from 1590s as "to think ill of."
forethought (n.): early 14c., "a thinking beforehand, the act of planning," verbal noun from forethink "think of something beforehand," from Old English foreþencan "to premeditate, consider;" see fore- + think. Meaning "prudence, provident care" is from 1719.
doublethink (n.): 1948, coined by Orwell in "Nineteen Eighty-Four," from double (adj.) + think.
unthinking (adj.): 1670s, "unreflecting;" 1680s, "heedless," from un- (1) "not" + present participle of think (v.).
methinks (v.): Old English me þyncð "it seems to me," from me (pron.), dative of I, + þyncð, third person singular of þyncan "to seem," reflecting the Old English distinction between þyncan "to seem" and related þencan "to think," which bedevils modern students of the language (see think). The two thinks were constantly confused, then finally merged, in Middle English. Related: Methought.
unthinkable (adj.): early 15c., "too large to be conceived, unimaginable," from un- (1) "not" + think (v.) + -able. Meaning "incapable of being framed by thought" is recorded from mid-15c.
aforethought (adj.): 1580s, from afore + past tense of think. Apparently an English loan-translation of Old French legalese word prepense (see prepense) in malice prepense "malice aforethought" (Coke).
thunk (v.): dialectal or jocular past tense or past participle of think, by 1876. Not historical, but by analogy of drink/drunk, sink/sunk, etc.
john (n.): "toilet," 1932, probably from jakes, used for "toilet" since 15c. Meaning "prostitute's customer" is from 1911, probably from the common, and thus anonymous, name by which they identified themselves. Meaning "policeman" is by 1901, from shortening of johndarm (1823), a jocular Englishing of gendarme.
"John Darm! who's he?" "What, don't you know!
In Paris he is all the go;
Like money here,--he's every thing;
A demigod--at least a king!
You cannot fight, you cannot drink,
Nor have a spree, nor hardly think,
For fear you should create a charm,
To conjure up the fiend John Darm!

["John Darm," in "Varieties in Verse," John Ogden, London, 1823]
twice (adv.): late Old English twies, from Old English twiga, twigea "two times," from Proto-Germanic *twiyes (source also of Old Frisian twia, Old Saxon tuuio), from PIE *dwis-, adverbial form of *dwo- "two" (see two). Spelling with -ce reflects the voiceless pronunciation.
Life is as tedious as a twice-told tale
Vexing the dull ear of a drowsy man.
["King John," III.iv.]
Think twice, then speak was an "old Prouerbe" by 1623. At twice, though less common than at once, means "at two distinct times; by two distinct operations."
guess (v.): c. 1300, gessen "to infer from observation, perceive, find out; form an opinion, judge, decide, discern; evaluate, estimate the number, importance, etc. of," perhaps from Scandinavian (compare Middle Danish gitse, getze "to guess," Old Norse geta "guess, get"), or from or influenced by Middle Dutch gessen, Middle Low German gissen "to guess," all from Proto-Germanic *getan "to get" (see get (v.)). The prehistoric sense evolution then would be from "get," to "take aim at," to "to estimate." Meaning "to hit upon the right answer" is from 1540s. Spelling with gu- is late 16c., sometimes attributed to Caxton and his early experience as a printer in Bruges. Related: Guessed; guessing. Guessing game attested from 1650s. To keep (someone) guessing "keep him in a state of suspense" is from 1896, American English.
[T]he legitimate, English sense of this word is to conjecture; but with us, and especially in New England, it is constantly used in common conversation instead of to believe, to suppose, to think, to imagine, to fancy. [Bartlett, "Dictionary of Americanisms," 1848]
tooth (n.): Old English toð (plural teð), from Proto-Germanic *tan-thuz (source also of Old Saxon, Danish, Swedish, Dutch tand, Old Norse tönn, Old Frisian toth, Old High German zand, German Zahn, Gothic tunþus), from PIE *dent- "tooth" (source also of Sanskrit danta, Greek odontos, Latin dens, Lithuanian dantis, Old Irish det, Welsh dent). Plural form teeth is an instance of i-mutation.

The loss of -n- before spirants is regular in Old English, Old Frisian, and Old Saxon: compare goose (n.), five. Also thought, from stem of think; couth from the stem of can (v.1); us from *uns.

Application to tooth-like parts of other objects (saws, combs, etc.) first recorded 1520s. Tooth and nail as weapons is from 1530s. The tooth-fairy is attested from 1964.
Aden: place in southern Arabia, ultimately from Akkadian edinnu "plain," which some think also is the root of Biblical Eden.
value (v.): mid-15c., "estimate the value of," also "think highly of," probably from value (n.). Related: Valued, valuing.
cogitate (v.): late 16c., from Latin cogitatus, past participle of cogitare "to think" (see cogitation). Related: Cogitated; cogitating.
cogitative (adj.): late 15c., from Old French cogitatif (14c.), from Medieval Latin cogitativus, from Latin cogitare "to think" (see cogitation).
ideate (v.): c. 1600, "imagine, form ideas," from idea + -ate (2). From 1862 as "to think." Related: Ideated; ideating.
dianetics (n.): 1950, coined by U.S. writer L. Ron Hubbard (1911-1986), an alteration of dianoetic (1670s) "of or pertaining to thought," from Greek dianoetikos "of or for thinking; intellectual," from dianoetos, verbal adjective from dianoe-esthai "to think," from dia- "through" (see dia-) + noe-ein "to think, suppose."
mention (n.): c. 1300, "a note, reference," from Old French mencion "mention, memory, speech," from Latin mentionem (nominative mentio) "a calling to mind, a speaking of, a making mention," from root of Old Latin minisci "to think," related to mens (genitive mentis) "mind," from PIE root *men- (1) "think" (see mind (n.)).
premeditate (v.): 1540s, from pre- + meditate, or a back formation from premeditation, or else from Latin praemeditatus, past participle of praemeditari "to think over." Related: Premeditated; premeditating.
mean (v.1): "intend, have in mind," Old English mænan "to mean, intend, signify; tell, say; complain, lament," from West Germanic *mainijan (source also of Old Frisian mena "to signify," Old Saxon menian "to intend, signify, make known," Dutch menen, German meinen "think, suppose, be of the opinion"), from PIE *meino- "opinion, intent" (source also of Old Church Slavonic meniti "to think, have an opinion," Old Irish mian "wish, desire," Welsh mwyn "enjoyment"), perhaps from root *men- "think" (see mind (n.)). Conversational question you know what I mean? attested by 1834.
heterodox (adj.): "not in accordance with established doctrines," 1630s, from Greek heterodoxos "of another or different opinion," from heteros "the other" (see hetero-) + doxa "opinion," from dokein "to appear, seem, think" (see decent).
pensive (adj.): late 14c., from Old French pensif "thoughtful, distracted, musing" (11c.), from penser "to think," from Latin pensare "weigh, consider," frequentative of pendere "weigh" (see pendant). Related: Pensively; pensiveness.
Docetism (n.): 1846, heresy of the Docetae, who held that the body of Jesus was a phantom, from Greek Doketai, name of the sect, literally "believers," from dokein "to seem, have the appearance of, think," related to doxa (see decent).
assent (v.): c. 1300, from Old French assentir "agree; get used to" (12c.), from Latin assentare "to agree with," frequentative of assentire, from ad- "to" (see ad-) + sentire "to feel, think" (see sense (n.)). Related: Assented; assenting.
automaton (n.): 1610s, from Latin automaton (Suetonius), from Greek automaton, neuter of automatos "self-acting," from autos "self" (see auto-) + matos "thinking, animated, willing," from PIE *mn-to-, from root *men- "to think" (see mind (n.)).
incogitable (adj.): "unthinkable, inconceivable," 1520s, from Late Latin incogitabilis "unthinking; unthinkable," from in- "not" (see in- (1)) + cogitabilis "thinkable, conceivable," from stem of cogitare "to think" (see cogitation).
mun (v.): auxiliary verb, now archaic or dialectal, "must," c. 1200, from Old Norse monu, a future tense auxiliary verb ultimately meaning "to intend" and from the PIE root *men- "to think" (see mind (n.)).
Eumenides: Greek, literally "the well-minded ones," a euphemism of the Erinys; see eu- "well, good;" second element from Greek menos "spirit, passion," from PIE *men-es-, suffixed form of *men- (1) "to think" (see mind (n.)).
repute (v.): late 14c., from Middle French reputer (late 13c.) or directly from Latin reputare "to count over, reckon; think over" (see reputation). Related: Reputed; reputing.
deem (v.): Old English deman "to judge, condemn, think, compute," from root of dom (see doom (n.)). Originally "to pronounce judgment" as well as "to form an opinion." The two judges of the Isle of Man were called deemsters in 17c., a title formerly common throughout England and Scotland and preserved in the surname Dempster.
pansy (n.): mid-15c., from Middle French pensée "a pansy," literally "thought, remembrance," from fem. past participle of penser "to think," from Latin pensare "consider," frequentative of pendere "to weigh" (see pendant). So called because it was regarded as a symbol of thought or remembrance. Meaning "effeminate homosexual man" is first recorded 1929.
ratio (n.): 1630s, "reason, rationale," from Latin ratio "reckoning, numbering, calculation; business affair, procedure," also "reason, reasoning, judgment, understanding," from rat-, past participle stem of reri "to reckon, calculate," also "think" (see reason (n.)). Mathematical sense "relationship between two numbers" is attested from 1650s.
opine (v.): "express an opinion," mid-15c., from Middle French opiner (15c.) and directly from Latin opinari "have an opinion, be of opinion, suppose, conjecture, think, judge," perhaps related to optare "to desire, choose" (see option). Related: Opined; opining.
omen (n.): 1580s, from Latin omen "foreboding," from Old Latin osmen, of unknown origin; perhaps connected with the root of audire "to hear" [OED] or from PIE *o- "to believe, hold as true" (source also of Greek oiomai "I suppose, think, believe").
amnesia (n.): "loss of memory," 1786 (as a Greek word in English from 1670s), Modern Latin, coined from Greek amnesia "forgetfulness," from a-, privative prefix, "not" (see a- (3)) + stem from mnasthai "to recall, remember," related to mnemnon "mindful," mneme "memory;" from PIE root *men- "to think, remember" (see mind (n.)).
Andromeda: constellation, 1667 (earlier Andromece, mid-15c.); in classical mythology the daughter of Cepheus and Cassiopeia, from Greek, literally "mindful of her husband," from andros, genitive of aner "man" (see anthropo-) + medesthai "to be mindful of, think on," related to medea (neuter plural) "counsels, plans, devices, cunning" (and source of the name Medea).
Clytaemnestra: also Clytemnestra, wife and murderess of Agamemnon, from Greek Klytaimnestra, from klytos "celebrated, heard of" (see loud) + mnester "wooer, suitor," literally "willing to mind, mindful of," related to mnasthai "to remember," from PIE root *men- "to think" (see mind (n.)).
dogma (n.): c. 1600 (in plural dogmata), from Latin dogma "philosophical tenet," from Greek dogma (genitive dogmatos) "opinion, tenet," literally "that which one thinks is true," from dokein "to seem good, think" (see decent). Treated in 17c.-18c. as a Greek word in English.
phrase (n.): 1520s, "manner or style of expression," also "group of words with some unity," from Late Latin phrasis "diction," from Greek phrasis "speech, way of speaking, enunciation, phraseology," from phrazein "to express, tell," from phrazesthai "to consider," from PIE *gwhren- "to think" (see frenetic). The musical sense of "short passage" is from 1789.