Online Etymology Dictionary

sink (v.): Old English sincan "become submerged, go under" (past tense sanc, past participle suncen), from Proto-Germanic *senkwanan (cf. 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 supplants Middle English sench (cf. drink/drench) which died out 14c. Sinking fund is from 1724; sinker in fishing line sense is from 1844. Adjective phrase sink or swim is from 1660s. To sink without a trace is WWI military jargon, translating German spurlos versenkt.
sink (n.): early 15c., "pool or pit for wastewater or sewage," from sink (v.). Sense of "shallow basin with drainpipe" first recorded 1560s.
sinkhole (n.): also sink-hole, as a geological phenomenon, "hole made in the earth by underground erosion," 1780, from sink (v.) + hole (n.).
Sinn Fein (n.): 1905, from Irish, literally "we ourselves," from Old Irish féin "self," from PIE *swei-no-, suffixed form of root *s(w)e- (see idiom). Movement founded 1905 by Irish journalist and politician Arthur Griffith (1872-1922).
sinner (n.): early 14c., agent noun from sin (v.).
Sino-: before vowels Sin-, word-forming element meaning "Chinese," 1879, from Late Latin Sinæ (plural) "the Chinese," from Ptolemaic Greek Sinai, from Arabic Sin "China," probably from Chinese Ch'in, name of the fourth dynasty of China (see China).
Sinologist (n.): 1814; see Sino- + -logy + -ist. Related: Sinology (1834).
sinsemilla (n.): potent strain of marijuana, 1975, from Mexican Spanish, literally "without seed."
sinter (n.): 1780, from German Sinter, cognate with English cinder.
sinuous (adj.): "full of turns and curves," 1570s, from Latin sinuosus "full of folds or bendings," from sinus "curve, fold, bend." Related: Sinuously; sinuousness.
sinus (n.): 1590s, "abscess, sore," from Medieval Latin sinus, from Latin sinus "bend, fold, curve." Meaning "hollow curve or cavity in the body" is attested from 1670s.
sinusitis (n.): "inflammation of the sinuses," 1896; see sinus + -itis.
sinusoid: 1823 in mathematics; 1900 in physiology, from sinus + -oid. Related: Sinusoidal.
Siouan (adj.): 1885, from Sioux + -an. Replacing Dakotan.
Sioux: group of North American Indian tribes, 1761, from North American French, short for Nadouessioux, sometimes said to be from Ojibway (Algonquian) Natowessiwak (plural), literally "little snakes," from nadowe "Iroquois" (literally "big snakes"). Another explanation traces it to early Ottawa (Algonquian) singular /na:towe:ssi/ (plural /na:towe:ssiwak/) "Sioux," apparently from a verb meaning "to speak a foreign language" [Bright]. In either case, a name given by their neighbors; the people's name for themselves is Dakota, literally "allies."
sip (v.): late 14c., perhaps related to Low German sippen "to sip," or Old English supan "to take into the mouth a little at a time" (see sup (2)). Related: Sipped; sipping.
sip (n.): 1630s, from sip (v.).
siphon (n.): 1650s, from French siphon (early 17c.), from Latin sipho (genitive siphonis), from Greek siphon "pipe, tube," of unknown origin.
siphon (v.): 1859, from siphon (n.). Figurative sense of "to draw off, divert" is recorded from 1940. Related: Siphoned; siphoning.
sir: c.1300, title of honor of a knight or baronet (until 17c. also a title of priests), variant of sire, originally used only in unstressed position. Generalized as a respectful form of address by mid-14c.; used as a salutation at the beginning of letters from early 15c.
sire (v.): "to beget, to be the sire of," 1610s, from sire (n.). Related: Sired; siring.
sire (n.): c.1200, title placed before a name and denoting knighthood, from Old French sire, from Vulgar Latin *seior, from Latin senior "older, elder" (see senior). Standing alone and meaning "your majesty" it is attested from early 13c. General sense of "important elderly man" is from mid-14c.; that of "father, male parent" is from mid-13c.
siren (n.): mid-14c., "sea nymph who by her singing lures sailors to their destruction," from Old French sereine, from Late Latin Sirena, from Latin Siren, from Greek Seiren ["Odyssey," xii.39 ff.], perhaps literally "binder," from seira "cord, rope."

Meaning "device that makes a warning sound" (on an ambulance, etc.) first recorded 1879, in reference to steamboats. Figurative sense of "one who sings sweetly and charms" is recorded from 1580s.
Sirius (n.): brightest star, late 14c., from Greek Seirios, literally "scorching." Probably so called from its ancient heliacal rising at the summer solstice (see dog days). An Egyptian name for it was Sothis.
Homer made much of it as [Kyon], but his Dog doubtless was limited to the star Sirius, as among the ancients generally till, at some unknown date, the constellation was formed as we have it, -- indeed till long afterwards, for we find many allusions to the Dog in which we are uncertain whether the constellation or its lucida is referred to. [Richard Hinckley Allen, Canis Major in "Star Names and Their Meanings," London: 1899]
Also cf. dog star.
sirloin (n.): early 15c., surloine, from Middle French surlonge, literally "upper part of the loin," from sur "over, above" + longe "loin," from Old French loigne (see loin).

English spelling with sir- dates from 17c., supposedly because the cut of beef was "knighted" by an English king for its superiority, a tale variously told of Henry VIII, James I, and Charles II, though none is chronologically possible.
sirocco (n.): "hot wind blowing from the Libyan deserts," 1610s, from Italian sirocco, from vulgar Arabic shoruq "the east wind," from Arabic sharqi "eastern, east wind," from sharq "east," from sharaqa "to rise" (in reference to the sun).
sirrah: 1520s, term of address used to men or boys expressing anger or contempt, archaic extended form of sir (in U.S., siree, attested from 1823).
sis (n.): 1650s, abbreviated form of sister; in American English, applied generally to girls and young women (1859).
sis-boom-bah: cheerleading chant, originally (1867) an echoic phrase imitating the sound of a skyrocket flight (sis), the burst of the fireworks (boom), and the reaction of the crowd ((b)ah).
sisal (n.): 1843, from Sisal, port in Yucatan, from which the rope-making fiber was exported.
sissy (n.): 1846, "sister," extended form of sis (q.v.). Meaning "effeminate man" is recorded from 1887; the adjective in this sense is from 1891. Sissy bar is recorded from 1969.
sist: legal term, from Latin sistere "to cause to stand" (see assist).
sister (n.): Old English sweostor, swuster, or a Scandinavian cognate (Old Norse systir, Swedish syster, Danish søster), in either case from Proto-Germanic *swestr- (cf. Old Saxon swestar, Old Frisian swester, Middle Dutch suster, Dutch zuster, Old High German swester, German Schwester, Gothic swistar),

These are from PIE *swesor, one of the most persistent and unchanging PIE root words, recognizable in almost every modern Indo-European language (e.g. Sanskrit svasar-, Avestan shanhar-, Latin soror, Old Church Slavonic, Russian sestra, Lithuanian sesuo, Old Irish siur, Welsh chwaer, Greek eor).

Probably from PIE roots *swe- "one's own" + *ser- "woman." For vowel evolution, see bury. Used of nuns in Old English; of a woman in general from 1906; of a black woman from 1926; and in the sense of "fellow feminist" from 1912.
sister-in-law (n.): mid-15c.; see sister + in-law.
sisterhood (n.): "state of being a sister," late 14c., from sister + -hood. Meaning "a society of sisters" (usually a religious order) is from 1590s; sense of "women having some common characteristic or calling" is from c.1600.
Sistine: 1769, literally "pertaining to Pope Sixtus," from Italian sistino, from Sixtus, name of five popes, from Latin sextus "sixth" (see Sextus). The "chapel" is named for Sixtus IV (Francesco della Rovere), pope 1471-84, who had it built.
sisyphean (adj.): "resembling the labors of Sisyphus," 1630s, from Sisyphus + -an.
Sisyphus: Greek Sisyphos, a name of unknown origin. King of Corinth, famed as "the craftiest of men," he was condemned in the afterlife to roll uphill a stone which perpetually rolls down again.
sit (v.): Old English sittan "to be seated, to seat oneself" (class V strong verb; past tense sæt, past participle seten), from Proto-Germanic *setjanan (cf. Old Saxon sittian, Old Norse sitja, Old Frisian sitta, Middle Dutch sitten, Dutch zitten, Old High German sizzan, German sitzen, Gothic sitan), from PIE root *sed- "to sit" (see sedentary).

In reference to a legislative assembly, from 1510s. Meaning "to baby-sit" is recorded from 1966. Sitting room first recorded 1771. Slang sitting duck "easy target" first recorded 1944; literal sense is from 1867 (it is considered not sporting to shoot at one). Sitting pretty is from 1916. To sit on one's hands was originally "to withhold applause" (1926); later, "to do nothing" (1959). To sit around "be idle, do nothing" is 1915, American English. To sit out "not take part" is from 1620s.
sit-in: 1936, in reference to session musicians; 1937, in reference to union action; 1941, in reference to student protests.
sit-up (n.): also situp, kind of physical exercise, 1955, from sit (v.) + up (adv.). Related: Sit-ups.
sitar (n.): 1845, from Hindi sitar, from Persian sitar "three-stringed," from si "three" (Old Persian thri-) + tar "string" (see tenet).
sitcom (n.): 1964, from sit(uation) com(edy), a phrase first attested 1953 (in a "TV Guide" article, with reference to "I Love Lucy").
Even Bing Crosby has succumbed to series TV and will appear in a sitcom as an electrical engineer who happens to break into song once a week. ["Life," Sept. 18, 1964]
site (n.): "place or position occupied by something," late 14c., from Anglo-French site, from Latin situs "place, position," from si-, root of sinere "let, leave alone, permit," of uncertain origin.
sith (adv., conj., prep.): Middle English, reduced from Old English siððan "then, thereupon; continuously, during which; seeing that," from *sið þon "subsequent to that."
situ: see in situ.
situate (v.): 1530s, "to give a site to," from Medieval Latin situatus, past participle of situare "to place, locate," from Latin situs "place, position" (see site). Related: Situated; situating.
situation (n.): late 15c., "place, position, or location," from Medieval Latin situationem (nominative situatio), from Late Latin situatus, past participle of situare (see situate). Meaning "state of affairs" is from 1750; meaning "employment post" is from 1803. Situation ethics first attested 1955.
situational (adj.): 1903, from situation + -al. Related: Situationally. Situational ethics attested from 1969.
situs: Latin, "situation, position" (see site).