The Scottish family name McNown is classified as being of personal name origin and such family names may be derived from a parental first name or from the font name of the grandfather or indeed a more remote ancestor of the original bearer of the surname. With regard to the family name McNown, it is an anglicised form ofd the Gaeliuc "Mac Fhionghuin or Mac Fhionnghain", meaning "Son of Finguine", an ancient Gaelic personal name meaning "fair-born".
As to the origin of the McNown name is that the original name was NOWN. The Nowns' were Scottish, from Aberdeen, and were of a high social station. The story has it that one of the Nown sons (unnamed) married beneath his station and this resulted in his being disinherited by his father. In retaliation he changed his name to "Son of Nown" or McNOWN.
This latter bit of folklore may be the basis for the James line folklore that it was James himself who was disinherited because his father disapproved of his marriage.