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

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "crossbones":
   Azrael, Black Death, Calvary cross, Christogram, Death,
   Greek cross, Grim Reaper, Jerusalem cross, Latin cross,
   Maltese cross, Pale Death, Reaper, Russian cross, T, X,
   angel of death, ankh, avellan cross, chi, chi-rho, christcross,
   crisscross, cross, cross ancre, cross botonee, cross bourdonee,
   cross fitche, cross fleury, cross formee, cross fourchee,
   cross grignolee, cross moline, cross of Cleves, cross of Lorraine,
   cross patee, cross recercelee, cross-crosslet, crosslet, crucifix,
   cruciform, crux, crux ansata, crux capitata, crux decussata,
   crux gammata, crux immissa, crux ordinaria, dagger, ex, exing,
   fork cross, gammadion, inverted cross, long cross, memento mori,
   pale horse, pale rider, papal cross, pectoral cross, potent cross,
   rood, saltire, sickle of Death, skull, skull and crossbones,
   swastika, tau, that fell sergeant, that grim ferryman,
   trefled cross, voided cross, white cross