Webster's Revised Unabridged Dictionary · 1913

They

They (tā) , pronoun, plural

[Icelandic þeir they, properly nom. pl. masc. of , , þat, a demonstrative pronoun, akin to the English definite article, Anglo-Saxon , seó, eaet, nom. pl. . See That.]

The plural of he, she, or it. They is never used adjectively, but always as a pronoun proper, and sometimes refers to persons without an antecedent expressed.
Jolif and glad they went unto here [their] rest And casten hem [them] full early for to sail. — Chaucer
They of Italy salute you. — Heb. xiii. 24
Blessed are they which do hunger and thirst after righteousness. — Matt. v. 6

They is used indefinitely, as our ancestors used man, and as the French use on; as, they say (French on dit), that is, it is said by persons not specified.