skull and cross-bones

skull and cross-bones
picture of a skull above two crossed bones, once used on the flags of pirates' ships and now to warn of danger, eg on bottles of poison 骷髅画, 骷髅旗（旧时为海盗旗标志, 现作危险标志, 尤用於毒药瓶上）.

牛津英汉双解词典