Their is Norse, and so are they and them. English borrowed all three from Old Norse in the 1100s and 1200s, replacing the native Old English forms — hiera in this case. This is the most remarkable single fact about Norse influence on English: pronouns are the last thing a language borrows, because they belong to the innermost grammar, and English borrowed a whole set.
The reason it happened here is probably practical. The native forms had worn down until he, him, her and their plurals were hard to keep apart, and the Norse forms beginning with th- were usefully distinct. Their use with a singular subject — everyone brought their coat — is not a modern lapse: it is attested from about 1300, and the Oxford English Dictionary cites it in Fielding, Goldsmith, Sydney Smith and Thackeray. Theirs is a double possessive, and the dialect form theirn is recorded from 1836.
their (pron.) plural possessive pronoun, "of, belonging to, or pertaining to them," c. 1200, from Old Norse þierra "of them," genitive of plural personal and demonstrative pronoun þeir "they" (see they ). It replaced Old English hiera . As an adjective from late 14c. Its use with singular objects, historically scorned by grammarians, is attested from c. 1300, and OED (1989) quotes this in Fielding, Goldsmith, Sydney Smith, and Thackeray. Theirs (c. 1300) is a double possessive. Alternative form theirn (1836) is attested in Midlands and southern dialect in U.K. and the Ozarks region of the U.S. There is a Middle English theiren from early 15c. also from c. 1200
their came in through Old Norse, and its spelling follows that family's conventions.
What gives it away on the page
| core vocabulary that isn't Old English | Borrowing pronouns is rare and marks deep contact, not fashion.they, them, their, are, take, want, die, knife, window, law |
Old Norse in the spelling guide · guess another word's family