A Keps nut, also called a K-nut or washer nut, is a nut

Nut (hardware)

A nut is a type of hardware fastener with a threaded hole. Nuts are almost always used opposite a mating bolt to fasten a stack of parts together. The two partners are kept together by a combination of their threads' friction, a slight stretch of the bolt, and compression of the parts...

 with an attached, free-spinning washer

Washer (hardware)

A washer is a thin plate with a hole that is normally used to distribute the load of a threaded fastener, such as a screw or nut. Other uses are as a spacer, spring , wear pad, preload indicating device, locking device, and to reduce vibration...

. It is used to make assembly more convenient. Common washer types are star-type lock washers, conical, and flat washers.

Keps is a trademark of ITW Shakeproof. The name comes from "kep" in Shakeproof, and the "s" is because usually more than one are purchased.