Locknut

A locknut is a nut that is fitted into a piece of pipe and screwed onto another pipe to join the two pieces. Locknuts, also  known as a lock nut, locking nut, prevailing torque nut, stiff nut or elastic stop nut, is a nut that resists loosening under vibrations and torque. The first type of locknut used was a fiber instead of nylon and was invented in 1931.