Possibly an abbreviation of Magpie (q.v.). An English slang name for a half Penny. It is thus defined by G. Parker, in Life's Painter, 1781 (p. 129).
Dickens, in Bleak House (xxiii.), uses the phrase "It can't be worth a mag to him," and Henry Kingsley, in Ravenshoe (i. 9) says, "As long as lie had a mag to bless himself with, he would always be a lazy, useless humbug." See Maggy Robb.
Source: Frey's Dictionary (American Journal of Numismatics, Vol. 50, 1916)