E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
i.e. Mac. A Gaelic prefix meaning son. (Gothic, magus, a son; Sanskrit, mah, to grow; Welsh, magu, to breed.) The Welsh ap is Mac changed to Map, and contracted into ap or p, as Apadam (Ap Adam), Prichard (P Richard).