According to Jamieson, Etymological Dictionary of the Scottish Language, this was a popular name for a bad half Penny. He de- tines Magg as a cant term for a half Penny with the plural Maggs. The latter word is used in Lothian to designate the gratuity which servants expect from those to whom they carry any goods.
The same authority cites an Aberdeenshire saying, "He's a very guid man, but I trow he's gotten a Maggy Rob o' a wife."
Synonyms: Maggie Rab
Source: Frey's Dictionary (American Journal of Numismatics, Vol. 50, 1916)