I was just having this conversation with a friend. Where does the word "the" come from?
Let me introduce you to the wonderful site that is Etymonline:
late O.E. þe, nominative masculine form of the demonstrative pronoun and adjective. After c.950, it replaced earlier se (masc.), seo (fem.), þæt (neut.), and probably represents se altered by the þ- form which was used in all the masc. oblique cases (see below). O.E. se is from [Proto-Indo-European] base **so-* "this, that" (cf. Skt. sa, Avestan ha, Gk. ho, he "the," Ir., Gael. so "this"). For the þ- forms, see that. The s- forms were entirely superseded in English by mid-13c., excepting dialectal survival slightly longer in Kent. O.E. used 10 different words for "the" (see table, below), but did not distinguish "the" from "that." That survived for a time as a definite article before vowels (cf. that one or that other). Adv. use in the more the merrier, the sooner the better, etc. is a relic of O.E. þy, originally the instrumentive case of the neuter demonstrative þæt (see that).
See also my comment on this related question: How are sentences like “the longer X, the more Y” called and can they be used in formal written English?