poss.pron. (attrib.) 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?).
[ ME f. ON their(r)a of them, genit. pl. of s THE, THAT ]