Them is Norse, borrowed in the late 1100s from Old Norse þeim, the dative of the plural pronoun that also gave English they and their. It replaced the native Old English forms him and heom.
Borrowing a pronoun is close to unheard of, and English borrowed the whole plural set. The reason is legible in what it displaced: the Old English plural was him, which had become indistinguishable from the singular masculine him after centuries of sound change. A language can tolerate a lot of ambiguity, but not that, and the Norse forms beginning with th- were conveniently distinct and fitted the pattern of the, that and this. Notably, them has been used of a single person of unknown gender since at least 1742 — the usage is older than most of the objections to it.
them (pron.) third person plural pronoun, late 12c., them , theim , from Old Norse þeim , dative of plural personal and demonstrative pronoun þeir (see they ). Replaced Old English cognate him , heom ; for explanation see they . Used of single persons of unknown gender by 1742. also from late 12c.
them 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