The Collaborative International Dictionary of English v.0.48:

locknut \locknut\, lock nut \lock nut\n.
   a supplementary nut that is screwed down on a primary nut to
   prevent it from loosening; a check nut.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

locknut
    n 1: supplementary nut that is screwed down on a primary nut to
         prevent it from loosening [syn: locknut, safety nut]