- theirs (pron.)
- possessive pronoun, "their own," early 14c., from their + possessive -s, on analogy of his, etc. In form, a double possessive.
- theirself (pron.)
- emphatic plural pronoun, c. 1300, from their + self, with self, originally an inflected adjective, treated as a noun with a meaning "person." Related: Theirselves.