Of uncertain ultimate origin. This was the name of the mother of St Augustine, as transmitted to us by her famous son. She was a citizen of Carthage, so her name may well be of Phoenician origin, but in the early Middle Ages it was taken to be a derivative of Latin monere ‘to warn, counsel’, since it was as a result of her guidance that her son was converted to Christianity.
English (very common in England, especially in the south Midlands, and in Wales) and German (especially northwestern Germany): patronymic from the personal name Adam. In the U.S. this form has absorbed many patronymics and other derivatives of Adam in languages other than English. (For forms, see Hanks and Hodges 1988.)