Etymology
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the 

definite article, late Old English þe, nominative masculine form of the demonstrative pronoun and adjective. After c.950, it replaced earlier se (masc.), seo (fem.), þæt (neuter), and probably represents se altered by the th- form which was used in all the masculine oblique cases.

Old English se is from PIE root *so- "this, that" (source also of Sanskrit sa, Avestan ha, Greek ho, he "the," Irish and Gaelic so "this"). For the þ- forms, see that. The s- forms were entirely superseded in English by mid-13c., excepting a slightly longer dialectal survival in Kent. Old English used 10 different words for "the," but did not distinguish "the" from "that." That survived for a time as a definite article before vowels (that one or that other).

Adverbial use in the more the merrier, the sooner the better, etc. is a relic of Old English þy, the instrumentive case of the neuter demonstrative (see that).

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thence (adv.)
late 13c., from Old English þanone, þanon "from that place" + adverbial genitive -es. Old English þanone/þanon is from Proto-Germanic *thanana (source also of Old Saxon thanana, Old Norse þana, Old Frisian thana, Old High German danana, German von dannen), related obscurely to the root of then, and ultimately from PIE demonstrative base *to- (see the). Written with -c- to indicate a voiceless "s" sound. Meaning "from that time" is from late 14c.; sense of "for that reason" is from 1650s. From thence is redundant.
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there (adv., conj.)
Old English þær "in or at that place, so far as, provided that, in that respect," from Proto-Germanic *thær (source also of Old Saxon thar, Old Frisian ther, Middle Low German dar, Middle Dutch daer, Dutch daar, Old High German dar, German da, Gothic þar, Old Norse þar), from PIE *tar- "there" (source also of Sanskrit tar-hi "then"), from root *to- (see the) + adverbial suffix -r.

Interjectional use is recorded from 1530s, used variously to emphasize certainty, encouragement, or consolation. To have been there "had previous experience of some activity" is recorded from 1877.
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then (adv.)
adverb of time, Old English þanne, þænne, þonne, from Proto-Germanic *thana- (source also of Old Frisian thenne, Old Saxon thanna, Dutch dan, Old High German danne, German dann), from PIE demonstrative pronoun root *to- (see the).

As a conjunction, "in that case, therefore," in Old English. As an adjective, "being at that time," from 1580s. As a noun from early 14c. For further sense development, see than. Similar evolutions in other Germanic languages; Dutch uses dan in both senses, but German has dann (adv.) "then," denn (conj.) "than." Now and then "at various times" is attested from 1550s; earlier then and then (c. 1200).
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thilk (pron., adj.)
"that same, the very thing," early 13c., contraction from þe "the" (see the) + ilce "same" (see ilk).
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nonce (n.)

in phrase for the nonce (Middle English for þe naness, c. 1200) "for a special occasion, for a particular purpose," a misdivision (see N for other examples) of for þan anes "for the once," in reference to a particular occasion or purpose, the þan being an altered form of the Middle English dative definite article þam (see the). The phrase was used from early 14c. as an empty filler in metrical composition.

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lest (conj.)
c. 1200, "that not," especially "for fear that" [OED calls it a negative particle of intention], from a contraction of the Old English phrase þy læs þe "the less that," from þy, instrumental case of demonstrative article þæt "that" + læs (see less) + conjunction þe (see the). The þy was dropped and the remaining two words contracted into early Middle English leste.
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hoi polloi (n.)
1837, from Greek hoi polloi (plural) "the people," literally "the many" (plural of polys, from PIE root *pele- (1) "to fill"). Used in Greek by Dryden (1668) and Byron (1822), in both cases preceded by the, even though Greek hoi means "the," a mistake repeated often by subsequent writers who at least have the excuse of ignorance of Greek. Ho "the" is from PIE *so- "this, that" (nominative), cognate with English the and Latin sic. From the adjective agoraios "pertaining to the agora; frequenting the market" Greek had hoi agoraioi "loungers in the market, loafers, common, low men."
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she (pron.)
mid-12c., probably evolving from Old English seo, sio (accusative sie), fem. of demonstrative pronoun (masc. se) "the," from PIE root *so- "this, that" (see the). The Old English word for "she" was heo, hio, however by 13c. the pronunciation of this had converged by phonetic evolution with he "he," which apparently led to the fem. demonstrative pronoun being used in place of the pronoun (compare similar development in Dutch zij, German sie, Greek he, etc.). The original h- survives in her. A relic of the Old English pronoun is in Manchester-area dialectal oo "she." As a noun meaning "a female," she is attested from 1530s.
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theco- 

before vowels thec-, word-forming element used in botany and zoology with the sense "case, capsule," from Latinized combining form of Greek thēkē "case, receptacle," from PIE *dhek-, suffixed form of root *dhe- "to set, put."

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