English, Welsh, Scottish, and Irish (of Norman origin): habitational name from Mortemer in Seine-Maritime, France, so called from Old French mort(e) ‘dead’ + mer ‘sea’ (Latin mare). The place name probably referred to a stagnant pond or partly drained swamp; there may also have been an allusion to the Biblical Dead Sea seen by crusaders. The Norman surname was taken to Ireland from England in the medieval period, where it has also been adopted by bearers of the Gaelic surnames Mac Muircheartaigh and ÓMuircheartaigh, commonly Anglicized as McMurty and Mortagh. Compare McMurdo.