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 The s- forms were entirely superseded in English by mid-13c., excepting a slightly longer dialectal survival in Kent.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).
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.
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).
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.