From Longman Dictionary of Contemporary Englishskull and crossbonesˌskull and ˈcrossbones noun [singular] 1 
TTWa picture of a human skull with two bones crossed below it, used in past times on the flags of pirate ships2 DANGEROUSa picture of a human skull with two bones crossed below it, used on containers to show that what is inside is poison or very dangerous
TTWa picture of a human skull with two bones crossed below it, used in past times on the flags of pirate ships2 DANGEROUSa picture of a human skull with two bones crossed below it, used on containers to show that what is inside is poison or very dangerousExamples from the Corpus
skull and crossbones• All of the drums were marked with a little skull and crossbones symbol.• Remember the cigarettes with the skull and crossbones on the pack? 