Dictionary.net

• A symbol of death, consisting of two human thigh or arm bones, placed crosswise, and often found on old monuments, etc., generally in conjunction with a skull.

1899 - The american dictionary of the english language.
By Daniel Lyons

• Figure of two thighbones laid across each other, usu. under skull as emblem of death.

1919 - The concise Oxford dictionary of current English
By Sir Augustus Henry