1. a nut specially constructed to prevent its coming loose, usually having a means of providing extra friction between itself and the screw.
2. Also called jam nut. a thin supplementary nut screwed down upon a regular nut to prevent its loosening. See illus. under nut. Also,lock'nut".
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.