lock washer

from WordNet (r) 3.0 (2006)

lock washer
    n 1: washer that prevents a nut from loosening [syn: {lockring},
         {lock ring}, {lock washer}]

from The Collaborative International Dictionary of English v.0.48

Lock washer \Lock" wash`er\, n.
   A washer that is not completely smooth or flat, designed to
   prevent a nut from loosening, placed under the nut on a screw
   or bolt.
   [1913 Webster]