Source : Webster's Revised Unabridged Dictionary (1913)

Crossbones \Cross"bones`\ (-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.

         Crossbones, scythes, hourglasses, and other lugubrios
         emblems of mortality.                    --Hawthorne.

Source : WordNet®

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