their Look up their at Dictionary.com
c.1200, from Old Norse þierra, genitive of þeir "they" (see they). Replaced Old English hiera. Use with singular objects, scorned by grammarians, is attested from c.1300. 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.