They
Middle English: from Old Norse their, nominative plural masculine of sá ; related to them and their, also to that and the.
wiktionary
From Middle English thei, borrowed in the 1200s from Old Norse þeir, [1] plural of the demonstrative sá which acted as a plural pronoun. Displaced native Old English hīe — which vowel changes had left indistinct from hē(“he”) — by the 1400s, [1] [2] [3] being readily incorporated alongside native words beginning with the same sound ( the, that, this). Used as a singular pronoun since 1300, [1] e.g. in the 1325 Cursor Mundi.
The Norse term (whence also Icelandic þeir(“they”), Faroese teir(“they”), Danish de(“they”), Swedish de(“they”), Norwegian Nynorsk dei(“they”)) is from Proto-Germanic *þai(“those”) (from Proto-Indo-European *to-(“that”)), whence also Old English þā(“those”) (whence obsolete English tho), Scots thae, thai, thay(“they; those”).
The origin of the determiner they(“the, those”) is unclear. The OED, English Dialect Dictionary and Middle English Dictionary [4] define it and its Middle English predecessor thei as a demonstrative determiner or adjective meaning "those" or "the". This could be a continuation of the use of the English pronoun they's Old Norse etymon þeir as a demonstrative meaning "those", but the OED and EDD say it is limited to southern, especially southwestern, England, specifically outside the region of Norse contact.
From earlier the'e, from there.
etymonline
they (pron.)
c. 1200, from a Scandinavian source (Old Norse þeir, Old Danish, Old Swedish þer, þair), originally masculine plural demonstrative pronoun, from Proto-Germanic *thai, nominative plural pronoun, from PIE *to-, demonstrative pronoun (see that). Gradually replaced Old English hi, hie, plurals of he, heo "she," hit "it" by c. 1400. Colloquial use for "anonymous people in authority" is attested from 1886. They say for "it is said" is in Milton.
The most important importation of this kind [from Scandinavian to English] was that of the pronomial forms they, them and their, which entered readily into the system of English pronouns beginning with the same sound (the, that, this) and were felt to be more distinct than the old native forms which they supplanted. Indeed these were liable to constant confusion with some forms of the singular number (he, him, her) after the vowels has become obscured, so that he and hie, him and heom, her (hire) and heora could no longer be kept easily apart. [Jespersen, "Growth and Structure of the English Language"]
