Online Etymology Dictionary

simultaneity (n.): 1650s, from simultaneous + -ity.
simultaneous (adj.): 1650s, from Medieval Latin simultaneus, perhaps from simultim "at the same time," extended from Latin simul "at the same time" (see similar (adj.)), or from simul with ending abstracted from Late Latin spontaneus, where the -t- is organic. Related: Simultaneously.
simurgh (n.): monstrous bird, rational and ancient, in Persian mythology, 1786, from Persian simurgh, from Pahlavi sin "eagle" + murgh "bird." Compare Avestan saeno merego "eagle," Sanskrit syenah "eagle," Armenian cin "kite." Probably identical with the roc (q.v.).
sin (v.): Old English syngian "to commit sin, transgress, err," from synn (see sin (n.)); the form influenced by the noun. Compare Old Saxon sundion, Old Frisian sendigia, Middle Dutch sondighen, Dutch zondigen, Old High German sunteon, German sündigen "to sin." Form altered from Middle English sunigen by influence of the noun.
sin (n.): Old English synn "moral wrongdoing, injury, mischief, enmity, feud, guilt, crime, offense against God, misdeed," from Proto-Germanic *sun(d)jo- "sin" (source also of Old Saxon sundia, Old Frisian sende, Middle Dutch sonde, Dutch zonde, German Sünde "sin, transgression, trespass, offense," extended forms), probably ultimately "it is true," i.e. "the sin is real" (compare Gothic sonjis, Old Norse sannr "true"), from PIE *snt-ya-, a collective form from *es-ont- "becoming," present participle of root *es- "to be" (see is).

The semantic development is via notion of "to be truly the one (who is guilty)," as in Old Norse phrase verð sannr at "be found guilty of," and the use of the phrase "it is being" in Hittite confessional formula. The same process probably yielded the Latin word sons (genitive sontis) "guilty, criminal" from present participle of sum, esse "to be, that which is." Some etymologists believe the Germanic word was an early borrowing directly from the Latin genitive. Also see sooth.

Sin-eater is attested from 1680s. To live in sin "cohabit without marriage" is from 1838; used earlier in a more general sense. Ice hockey slang sin bin "penalty box" is attested from 1950.
Sinai: the mountain is perhaps named for Sin, a moon goddess worshipped by Sumerians, Akkadians, and ancient Arabs. As an adjectival form, Sinaic (1769), Sinaitic (1786).
since (adv.): early 15c., synnes, from sithenes "since," from sithen (plus adverbial genitive -es), from Old English siððan "afterward, from now on, hereafter, further, later, as soon as, after that," originally sið ðan "after that," from sið "after" (see sith) + ðan, weakened form of ðam, dative of ðæt (see that).

As a conjunction from late 14c.; as a preposition from 1510s; "from the time when," hence "as a consequence of the fact that." Modern spelling replaced syns, synnes 16c. to indicate voiceless final -s- sound. Since when? often expressing incredulity, is from 1907.
sincere (adj.): 1530s, "pure, unmixed," from Middle French sincere (16c.), from Latin sincerus, of things, "whole, clean, pure, uninjured, unmixed," figuratively "sound, genuine, pure, true, candid, truthful," of uncertain origin. The ground sense seems to be "that which is not falsified." Meaning "free from pretense or falsehood" in English is from 1530s.

There has been a temptation to see the first element as Latin sine "without." But there is no etymological justification for the common story that the word means "without wax" (*sin cerae), which is dismissed out of hand by OED and others, and the stories invented to justify that folk etymology are even less plausible. Watkins has it as originally "of one growth" (i.e. "not hybrid, unmixed"), from PIE *sm-ke-ro-, from *sem- "one" (see same) + root of crescere "to grow" (see crescent). De Vaan finds plausible a source in a lost adjective *caerus "whole, intact," from a PIE root meaning "whole."
sincerely (adv.): 1530s, "correctly;" 1550s, "honestly," from sincere + -ly (2). As a subscription to letters, recorded from 1702.
sincerity (n.): early 15c., "honesty, genuineness," from Middle French sinceritie (early 16c., Modern French sincérité) and directly from Latin sinceritatem (nominative sinceritas) "purity, soundness, wholeness," from sincerus "whole, clean, uninjured," figuratively "sound, genuine, pure, true, candid, truthful" (see sincere).
sinciput (n.): "forepart of the head," 1570s, from Latin sinciput "half a head," also "one of the smoked cheeks of a pig," from semi- (see semi-) + caput (see capitulum. Related: Sincipital.
sine (n.): trigonometric function, 1590s (in Thomas Fale's "Horologiographia, the Art of Dialling"), from Latin sinus "fold in a garment, bend, curve, bosom" (see sinus). Used mid-12c. by Gherardo of Cremona in Medieval Latin translation of Arabic geometrical text to render Arabic jiba "chord of an arc, sine" (from Sanskrit jya "bowstring"), which he confused with jaib "bundle, bosom, fold in a garment."
sine die: "indefinitely," Latin, literally "without (fixed) day," from sine "without" (see sans) + ablative singular of dies "day" (see diurnal).
sine prole: legalistic Latin, "without issue," from sine "without" (see sans) + prole, ablative of proles "offspring" (see prolific).
sine qua non: "an indispensable condition," Latin, literally "without which not," from sine "without" (see sans) + qua ablative fem. singular of qui "which" (see who) + non "not" (see non-). Feminine to agree with implied causa. The Latin phrase is common in Scholastic use. Sometimes a masculine form, sine quo non, is used when a person is intended. Proper plural is sine quibus non.
sinecure (n.): 1660s, "church benefice with an emolument but without parish duties," from Medieval Latin beneficium sine cura "benefice without care" (of souls), from Latin sine "without" (see sans) + cura, ablative singular of cura "care" (see cure (n.1)).
sinew (n.): Old English seonowe, oblique form of nominative sionu "sinew," from Proto-Germanic *senawo (source also of Old Saxon sinewa, Old Norse sina, Old Frisian sine, Middle Dutch senuwe, Dutch zenuw, Old High German senawa, German Sehne), from PIE root *sai- "to tie, bind" (source also of Sanskrit snavah "sinew," Avestan snavar, Irish sin "chain").
sinewy (adj.): late 14c., "made of sinews," from sinew + -y (2). As "tough, stringy" from 1570s.
sinfonia (n.): 1773, from Italian sinfonia, from Medieval Latin symphonia (see symphony).
sinful (adj.): Old English synnfull "full of sin, wicked, unholy, contrary to the laws of God;" see sin (n.) + -ful. Weakened sense of "contrary to propriety or decency" is from 1863. Related: Sinfully; sinfulness.
sing (v.): Old English singan "to chant, sing, celebrate, or tell in song," also used of birds (class III strong verb; past tense sang, past participle sungen), from Proto-Germanic *sengwan (source also of Old Saxon singan, Old Frisian sionga, Middle Dutch singhen, Dutch zingen, Old High German singan, German singen, Gothic siggwan, Old Norse syngva, Swedish sjunga), from PIE root *sengwh- "to sing, make an incantation." The criminal slang sense of "to confess to authorities" is attested from 1610s.

No related forms in other languages, unless perhaps it is connected to Greek omphe "voice" (especially of a god), "oracle;" and Welsh dehongli "explain, interpret." The typical Indo-European root is represented by Latin canere (see chant (v.)). Other words meaning "sing" derive from roots meaning "cry, shout," but Irish gaibim is literally "take, seize," with sense evolution via "take up" a song or melody.
sing (n.): "act of singing," especially collective, 1850, from sing (v.).
sing-along: 1959, noun and adjective, from verbal phrase; see sing (v.) + along (adv.). Originally associated with U.S. music producer Mitch Miller (1911-2010).
sing-song (adj.): also singsong, musically repetitive and unvarying, 1734, from earlier use as a noun meaning "a jingling ballad" (c. 1600), from sing (v.) + song (n.).
Singapore: from Sanskrit Simhapuram "Lion City," from simhah "lion" + puram "city," from PIE *pele- "citadel, fortified high place" (see polis (n.)). The name is perhaps metaphoric of something, as no lions are found there. Singapore sling attested from 1930; said on the island to have been invented there 1915 by a barman named Ngian Tong Dron.
singe (v.): Old English sengan "to burn lightly, burn the edges" (of hair, wings, etc.), from Proto-Germanic *sangjanan (source also of Old Frisian of-sendza, Middle Dutch singhen, Dutch zengen, Old High German sengan, German sengen "to singe"). The root is said to be related to that of sing (v.), on the idea of some sort of sound produced by singeing (Century Dictionary), but Klein's sources reject this. Related: Singed; singeing. Singed cat "person whose appearance does not do him justice, person who is better than he looks" is from 1827.
singer (n.): early 14c. (mid-13c. as a surname), agent noun from sing (v.). Old English had songer "psalm-writer," sangere "singer, poet" (also see songster).
Singh: common surname and middle name in North India, later (1699) adopted by Sikhs as a title after their initiation ceremony, also a surname adopted by male Sikhs; 1620s in English, from Hindi Singh, from Sanskrit simhah "lion."
single (n.): c. 1400, "unmarried person," mid-15c., "a person alone, an individual," from single (adj.). Given various technical meanings from 16c. Sports sense is attested from 1851 (cricket), 1858 (baseball). Of single things from 1640s. Meaning "one-dollar bill" is from 1936. Meaning "phonograph record with one song on each side" is from 1949. Meaning "unmarried swinger" is from 1964; singles bar attested from 1969. An earlier modern word for "unmarried or unattached person" is singleton (1937).
single (adj.): early 14c., "unmarried," from Old French sengle, sangle "alone, unaccompanied; simple, unadorned," from Latin singulus "one, one to each, individual, separate" (usually in plural singuli "one by one"), from sim- (stem of simplus; see simple) + diminutive suffix. Meaning "consisting of one unit, individual, unaccompanied by others" is from late 14c. Meaning "undivided" is from 1580s. Single-parent (adj.) is attested from 1966.
single (v.): "to separate from the herd" (originally in deer-hunting, often with forth or out), 1570s, from single (adj.). Baseball sense of "to make a one-base hit" is from 1899 (from the noun meaning "one-base hit," attested from 1858). Related: Singled; singling.
single-handed (adj.): 1709, "done alone," from single (adj.) + -handed. Meaning "using one hand only" is from 1844. Related: Single-handedly.
single-minded (adj.): 1570s, "sincere, honest" (a sense also in single-hearted); meaning "having a single aim or purpose" is from 1860. See single (adj.) + minded. Related: Single-mindedly; single-mindedness.
singlet (n.): "unlined woolen garment," c. 1746, from single (adj.) in clothing sense of "unlined, of one thickness" (late 14c.) + -et, apparently in imitation of doublet.
singleton (n.): "single card of a suit in a hand," 1876, originally in whist, from single (adj.); compare simpleton, etc. Extended early 20c. to other instances of singularity.
singly (adv.): c. 1300, from single (adj.) + -ly (2).
singspiel (n.): 1876, from German Singspiel, literally "a singing play," from singen "to sing" (see sing (v.)) + Spiel "a play" (see spiel). Kind of performance popular in Germany late 18c.
singular (adj.): mid-14c., "alone, apart; being a unit; special, unsurpassed," from Old French singuler "personal particular; distinctive; singular in number" (12c., Modern French singulier) or directly from Latin singularis "single, solitary, one by one, one at a time; peculiar, remarkable," from singulus (see single (adj.)). Meaning "remarkably good, unusual, rare, separated from others (by excellence), uncommon" is from c. 1400 in English; this also was a common meaning of Latin singularis.
singularity (n.): c. 1400, "unusual behavior," also "singleness of aim or purpose," from Old French singulerte "peculiarity" (12c., Modern French singularité) or directly from Late Latin singularitatem (nominative singularitas) "a being alone," from singularis (see singular (adj.)). Meaning "fact of being different from others" is c. 1500. Mathematical sense of "point at which a function takes an infinite value" is from 1893. Astronomical use is from 1965.
singularly (adv.): late 14c., "exclusively, alone, solely; uniquely; individually; in an unusual way, especially," from singular + -ly (2).
singultus (n.): Latin, "a sob; a speech broken by sobs."
Sinhalese (adj.): also Singhalese, "pertaining to Sri Lanka," 1797, from Sanskrit Sinhala "Sri Lanka, Ceylon," from simhala-, literally "of lions," from simhah "lion." As the name of a language spoken there, it is attested from 1801.
Sinic (adj.): "Chinese," 1660s, from Medieval Latin Sinicus, from Sina "China," from Late Latin Sinae (plural) "the Chinese" (see Sino-).
sinical (adj.): "of or relating to sines," 1590s, from sine + -ical.
Sinicism (n.): "Chinese ways, Chinese affectations," 1891; see Sino- + -ism. Related: Sinicize; Sinification.
sinister (adj.): early 15c., "prompted by malice or ill-will, intending to mislead," from Old French senestre, sinistre "contrary, false; unfavorable; to the left" (14c.), from Latin sinister "left, on the left side" (opposite of dexter), of uncertain origin. Perhaps meaning properly "the slower or weaker hand" [Tucker], but Klein and Buck suggest it's a euphemism (see left (adj.)) connected with the root of Sanskrit saniyan "more useful, more advantageous." With contrastive or comparative suffix -ter, as in dexter (see dexterity).

The Latin word was used in augury in the sense of "unlucky, unfavorable" (omens, especially bird flights, seen on the left hand were regarded as portending misfortune), and thus sinister acquired a sense of "harmful, unfavorable, adverse." This was from Greek influence, reflecting the early Greek practice of facing north when observing omens. In genuine Roman auspices, the augurs faced south and left was favorable. Thus sinister also retained a secondary sense in Latin of "favorable, auspicious, fortunate, lucky."

Meaning "evil" is from late 15c. Used in heraldry from 1560s to indicate "left, to the left." Bend (not "bar") sinister in heraldry indicates illegitimacy and preserves the literal sense of "on or from the left side" (though in heraldry this is from the view of the bearer of the shield, not the observer of it).
sinistral (adj.): late 15c., "unlucky," from Old French senestral, sinistral or Medieval Latin *sinistralis, from sinister (see sinister). Meaning "on the left side" is from 1803. Related: Sinistrally.
sinistrorse (adj.): 1856, a word wanted by the botanists to describe the direction of spiral structures in nature, from Latin sinistrorsus "toward the left side," from sinister "left" (see sinister). It was paired with dextrorse but confusion over what was the proper point of view to reckon leftward or rightward spiraling prevented the word being as useful as it might have been.
sink (n.): early 15c., "cesspool, pit for reception of wastewater or sewage," from sink (v.). Figurative sense of "place where corruption and vice abound" is from 1520s. Meaning "drain for carrying water to a sink" is from late 15c. Sense of "shallow basin (especially in a kitchen) with a drainpipe for carrying off dirty water" first recorded 1560s. In science and technical use, "place where heat or other energy is removed from a system" (opposite of source), from 1855.
sink (v.): Old English sincan (intransitive) "become submerged, go under, subside" (past tense sanc, past participle suncen), from Proto-Germanic *senkwan (source also of Old Saxon sinkan, Old Norse sökkva, Middle Dutch sinken, Dutch zinken, Old High German sinkan, German sinken, Gothic sigqan), from PIE root *sengw- "to sink."

The transitive use (mid-13c.) supplanted Middle English sench (compare drink/drench) which died out 14c. Related: Sank; sunk; sinking. Sinking fund is from 1724. Adjective phrase sink or swim is from 1660s. To sink without a trace is World War I military jargon, translating German spurlos versenkt.