casual, generic term of address for a man, 1928, from Irish and Gaelic mac, a common element in Scottish and Irish names (literally "son of;" see Mac-); hence used generally from 1650s for "a Celtic Irishman."
Entries linking to mac
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, Macdonald, McDonald.
ap- 2
patronymic element in Welsh pedigrees and names, earlier map "son," cognate with Gaelic mac. Since 17c. merged into surnames and reduced to P- or B- (Ap Rhys = Price, Ap Evan = Bevan, Bowen = Ap Owen, etc.).
It is said that a Welshman who evidently was not willing to be surpassed in length of pedigree, when making out his genealogical tree, wrote near the middle of his long array of 'aps' — "about this time Adam was born." ["Origin and Significance of our Names," The Chautauquan, Oct. 1887-July 1888]