During the reign of David I. of Scotland, a younger brother of the chief of the powerful clan Chattan espoused the clerical life, and in due time became abbot of Kingussie. His elder brother died childless, and the chieftainship devolved on the abbot. He procured the needful dispensation from the Pope, married the daughter of the thane of Calder, and a swarm of little “Kingussies” was the result. The good people of Invernessshire called them the Mac-phersons, i.e. the sons of the parson.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894