Mac-

common conjoined prefix in Scottish and Irish names, from Old Celtic *makko-s "son." Cognate *makwos "son" produced Old Welsh map, Welsh mab, ap "son;" also probably cognate with Old English mago "son, attendant, servant," Old Norse mögr "son," Gothic magus "boy, servant," Old English mægð "maid" (see maiden).

Formerly often abbreviated to M' and followed by a capital letter, or spelled out Mac and then rarely used with a capital; as, M'Donald, Macdoland, McDonald.