1) Safety nut 2) Special sort of fastener 
a nut designed not to loosen due to vibration.
A locknut, 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. Elastic stop nuts and prevailing torque nuts are of the particular type where some portion of the nut deforms elastically to provide a locking action. The first type used fiber instead...
[n] - supplementary nut that is screwed down on a primary nut to prevent it from loosening
safety nut noun supplementary nut that is screwed down on a primary nut to prevent it from loosening