Skull symbol takes the shape of a human skull sometimes with a cross of bones. This symbol has first emerged in the late middle ages as a symbol of death, and people used to draw it on the tombstone. In the modern days, the symbol is used more to indicate a warning of danger. Skull symbol also could be found printed on chemical hazards, especially poisons substance and deadly chemical.
In the military, some people use it to indicate strength and warning of ferocity.
In the 14th century, it became associated with pirates as they used to draw this symbol on a flag and hang it on their ships.