1747, Scottish, of unknown origin, perhaps related to Gael. colpach "heifer, steer, colt;" colpa "cow, horse." The Lowland name of a demon in the shape of a horse that was reputed to haunt lakes and rivers and to delight in causing drownings. But unlike its equivalents in Dan. (nøkken) and Icel. (nykur), it was occasionally benevolent, especially to millers by keeping their streams running.