Patronymic was used by illegitimate royal children who were acknowledged as such by their fathers. The use of "Mac" in some form was prevalent in Scottish Gaelic, Irish and Manx, in all of which it denotes "son". "Mc" is also a frequent anglicisation in both Scotland and Ireland. In Ireland, the forms "Mag" and "M'" are encountered. The prefix "Mac" is used to form a patronym, such as "Mac Coinnich" – or the anglicized 'Mackenzie' – son of Coinneach/Kenneth. The female equivalent of "Mac" is "Nic", condensed from "nighean mhic" (in Scottish Gaelic) or "iníon mhic" (in Irish), both meaning daughter.
McKinley (surname) nickname given the deposed MacDunleavy dynasty royals while exiled in Tirconnell and elsewhere. Being, also, one of Ireland's ancient hereditary medical families, the MacDunleavy (variant English spelling MacDonlevy) were in Tirconnell accorded the high Gaelic status of "ollahm leighis" or the official physicians to the O'Donnell clan in County Donegal and practiced as physicians while exiled in Argyll, Scotland. The nickname was Mac an Leigh. Per this scenario the Gaelic language patronymic forming prefix "Mac" (meaning "descended of") is joined to the Gaelic language "Léigh" meaning leech, but denoting a physician. Leeching (medical) having been for millennia, in Gaelic Ireland