they

From Middle English þei, borrowed in the 1200s from Old Norse þeir, plural of the demonstrative sá which acted as a plural pronoun. Displaced native Middle English he from Old English hīe — which vowel changes had left indistinct from he (“he”) — by the 1400s, being readily incorporated alongside native words beginning with the same sound (the, that, this). Used as a singular pronoun since 1300, 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”), Swabian dia (“they”).

The origin of the determiner they (“the, those”) is unclear. The OED, English Dialect Dictionary and Middle English Dictionary 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.