A Palnut is a locking device for nuts made of stamped sheet metal, and is a registered trademark. The palnut is screwed on the bolt on top of an ordinary nut, and has a series of protruding barbs that locks the nut in place when the nut is tightened. The Palnut itself is reusable. A Palnut should not be used as a substitute for a positive locking device such as a castellated nut or nuts with safety wire.
References
Palnut Wikipedia(Text) CC BY-SA