the

definite article, late Old English þe, nominative masculine form of the demonstrative pronoun and adjective. After c. 950, it displaced earlier se (masc.), seo (fem.), þæt (neuter), and probably represents se altered by the th- form which was used in 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 The s- forms were superseded in English by mid-13c., with a slightly longer dialectal survival in Kent.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).

In adverbial use, in clauses such as the more the merrier, the first the is a different word, a fossil of Old English þy, the instrumentive case of the neuter demonstrative (see that), used with relative force: “by how much more ____, by so much more ____.” Of the common phrases, the sooner the better, is by 1771; the less said the better from 1680s.

In emphatic use, “the pre-eminent, par excellence, most approved or desirable,” by 1824, often italicized. With relations (the wife, etc.) by 1838.

Source: O
https://www.etymonline.com
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