Many Scottish surnames begin with the Gaelic word Mac, son, e. g. MacInnes, MacAllister. In Gaelic, the form changes when referring to a female, to Nic.

Murdo MacRae is Murchadh MacRath, but Margaret MacRae is Mairead NicRath (compare Icelandic, Magnusson, Magnusdottir).

The above is from

www.english.uiowa.edu/faculty/ boos/images/questions/gaelic.htm

or Google

scottish gaelic ghaidhlig