locknut

from 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}]

from 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]