Etymology
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sit (v.)
Old English sittan "to occupy a seat, be seated, sit down, seat oneself; remain, continue; settle, encamp, occupy; lie in wait; besiege" (class V strong verb; past tense sæt, past participle seten), from Proto-Germanic *setjan (source also of Old Saxon sittian, Old Norse sitja, Danish sidde, Old Frisian sitta, Middle Dutch sitten, Dutch zitten, Old High German sizzan, German sitzen, Gothic sitan), from PIE root *sed- (1) "to sit."

With past tense sat, formerly also set, now restricted to dialect, and sate, now archaic; and past participle sat, formerly sitten. In reference to a legislative assembly, from 1510s. Meaning "to baby-sit" is recorded from 1966.

To sit back "be inactive" is from 1943. 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 1650s. Sitting pretty is from 1916.
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tight (adj.)

c. 1400, tyght "dense, close, compact," from Middle English thight, from Old Norse þettr "watertight, close in texture, solid," and also from Old English -þiht (compare second element in meteþiht "stout from eating"), both from Proto-Germanic *thinhta- (source also of Middle High German dihte "dense, thick," German dicht "dense, tight," Old High German gidigan, German gediegen "genuine, solid, worthy"), from PIE root *tenk- (2) "to become firm, curdle, thicken" (source also of Irish techt "curdled, coagulated," Lithuanian tankus "close, tight," Persian tang "tight," Sanskrit tanakti "draws together, contracts").

Sense of "drawn, stretched" is from 1570s; meaning "fitting closely" (as of garments) is from 1779; that of "evenly matched" (of a contest, bargain, etc.) is from 1828, American English; that of "drunk" is from 1830. Of persons, "close, intimate, sympathetic" from 1956. From 1670s as an adverb; to sit tight is from 1738; sleep tight as a farewell in sending someone off to bed is by 1871. Related: Tightly; tightness. Tight-assed "unwilling to relax" is attested from 1903. Tight-laced is recorded from 1741 in both the literal and figurative senses. Tight-lipped is first attested 1872.

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skin-tight (adj.)
"fitting like skin," 1885, originally of men's clothing, from skin (n.) + tight (adj.).
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sit-in 
1936, in reference to session musicians; 1937, in reference to union action; 1941, in reference to student protests. From the verbal phrase; see sit (v.) + in (adv.). To sit in is attested from 1868 in the sense "attend, be present;" from 1919 specifically as "attend as an observer."
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sit-up (n.)
also situp, kind of physical exercise, 1955, from the verbal phrase (attested from early 13c.); see sit (v.) + up (adv.). Related: Sit-ups.
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sit-down (adj.)
1836 of meals, 1936 of strikes, from verbal phrase (c. 1200), from sit (v.) + down (adv.); as a noun, sit-down "act of sitting down" is from 1861.
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piezo- 

word-forming element meaning "pressure," from Greek piezein "to press tight, squeeze," from PIE *pisedyo- "to sit upon" (source also of Sanskrit pidayati "presses, oppresses"), from *pi "on," short for *epi (see epi-) + root *sed- (1) "to sit." First in piezometer (1820), an instrument for ascertaining or testing pressure. It was in common use in word formation from c. 1900.

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tighten (v.)
"to make tight," 1727; the earlier verb was simply tight, from Old English tyhtan, from the root of tight. Related: Tightened; tightening.
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airtight (adj.)
also air-tight, "impermeable to air," 1760, from air (n.1) + tight. Figurative sense of "incontrovertible" (of arguments, alibis, etc.) is from 1929.
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tights (n.)
1827, "tight-fitting breeches," from tight. Meaning "skin-tights worn by dancers, acrobats, etc." is attested from 1836.
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