Irish spelling (reflecting the traditional pronunciation) of the Scottish surname Colquhoun is a Scottish: habitational name from the barony of Colquhoun in Dumbartonshire. The name appears to derive from Gaelic còil, cùil ‘nook’, ‘corner’, or coill(e) ‘wood’ + cumhann ‘narrow’. The usual Scottish pronunciation is ka-hoon.

Source: Dictionary of American Family Names by Patrick Hanks Oxford University Press