From Middle English thei, borrowed in the 1200s from Old Norse þeir, nominative plural masculine of the demonstrative sá, which acted as a plural pronoun. It displaced native Old English hī, hīe (“they”) — which vowel changes had left indistinct from he (“he”) — by the 1400s, being readily incorporated alongside similar native words beginning with the same sound (the, that, this).
The Norse term (whence also Icelandic þeir (“they”), Faroese teir (“they”), Swedish de (“they”), Norwegian Nynorsk dei (“they”)) derives 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”).
Used as a singular pronoun since 1300, for example in the 1325 Cursor Mundi (which uses þer).
From earlier the'e, from there.
they was also found in the following language(s): Middle English