skull and crossbones

noun1

1. •
   A depiction of a human skull and two crossed femurs (thighbones), a symbol of death traditionally used on the Jolly Roger pirate flag, but now used on chemical containers and other hazardous items as an indicator of warning of toxicity or other life-threatening dangers or risks.

   • In an obscure corner of the town stands a house of extreme antiquity, over the door of which are still to be seen a skull and crossbones. — 1832