The Collaborative International Dictionary of English v.0.48:

Crossbones \Cross"bones`\ (kr[o^]s"b[=o]nz`), n. pl.
   A representation of two of the leg bones or arm bones of a
   skeleton, laid crosswise, often surmounted with a skull, and
   serving as a symbol of death.
   [1913 Webster]

         Crossbones, scythes, hourglasses, and other lugubrious
         emblems of mortality.                    --Hawthorne.
   [1913 Webster]

WordNet (r) 3.0 (2006):

crossbones
    n 1: two crossed bones (or a representation of two crossed
         bones) used as a symbol danger or death