From WordNet (r) 2.0 (August 2003) [wn]:
crossbones
n : two crossed bones (or a representation of two crossed bones)
used as a symbol danger or death
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]