skull and cross·bones 
/ˌskʌlənˈkrɑ:sˌboʊnz/ noun
/ˌskʌlənˈkrɑ:sˌboʊnz/ noun [singular] : a picture of a human skull above two crossed bones that was used on the flags of pirate ships in the past and is now used as a warning label on containers of poisonous or dangerous substances
