their Look up their at Dictionary.com
c.1200, from O.N. žierra, gen. of žeir "they" (see they). Replaced O.E. 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.