A skull and crossbones is a symbol consisting of a human skull and two bones crossed together under the skull. It is generally used as a warning for something that is dangerous or deadly, usually poison. 

The symbol, or some variation thereof, was also featured on the Jolly Roger, the traditional flag of European and American pirates. It is also used by the Skull and Bones, a secret society at Yale University.

History of the symbol

In 1829 New York State required the labeling of all containers of poisonous substances. The skull and crossbones symbol appears to have been used for that purpose since the 1850s. Previously a variety of motifs had been used, including  drawings of skeletons. 

In the 1870s Americans began using bright cobalt bottles with a variety of raised bumps and designs (to enable easy recognition in the dark) to indicate poison, but by the 1880s the skull and cross bones had become ubiquitous, and the brightly coloured bottles lost their association.

Uses

Today, the skull and crossbones is still the only standard symbol for poison. It is, however, less common outside industrial usage than it once was. Apart from its negative marketing effect on environmentally conscious consumers, it may actually attract children due to its association with pirates, a popular toy and play theme. For this reason, there has been a proposal to replace the skull and crossbones by the hopefully more meaningful "Mr. Yuk" symbol. 

The skull and crossbones is also used by the British Army. The 17th Lancers adopted the skull and crossbones as its cap badge upon its creation in 1759, becoming known as the 'Death or Glory Boys'. The cap badge was later used by the 17th/21st Lancers and its present-day successor, the Queen's Royal Lancers who are still nicknamed the 'Death or Glory Boys'. 

Some crucifixes feature a skull and crossbones beneath the corpus (the depiction of Jesus' body), in reference to a legend that the place of the crucifixion was also the burial place of Adam.

 .