skull and crossbones

Short definition from Wiktionary

Noun

1. 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.
2. 2
   Alternative form of skull and crossbones.