1 of or belonging to them or themselves (their house; their own business).
2 (Their) (in titles) that they are (Their Majesties).
3 disp. as a third person sing. indefinite meaning 'his or her' (has anyone lost their purse?).
Etymology: ME f. ON their(r)a of them, genit. pl. of saacute THE, THAT
Useful english dictionary. 2012.