skull and crossbones - definition and synonyms

noun [singular] 

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/ˌskʌl ən ˈkrɒsˌbəʊnz/
  1. a picture of two crossed bones under a skull. It was used in the past on ships belonging to pirates (=sailors that attacked and stole from other ships) and is now used for warning of danger, for example on a container that holds a harmful substance.