We had fun researching this one back in 2009. This from Wikipedia:

A skull and cross bones 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.

In 1829 New York State required the labelling 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 the Danish "+ + +" and drawings of skeletons.

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 (at least in the US) by the hopefully more meaningful and less enticing olive green "Mr. Yuk" symbol. However, Mr. Yuk and his graphic rendering are registered trademarks and service marks of his creator, the Children's Hospital of Pittsburgh of UPMC, and the rendering itself is additionally protected by copyright. This means that the name and graphic image cannot be used without a license from the owner, unlike the Skull and crossbones, which is in the public domain.

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