the.com/skull and crossbones

means A symbol of two crossed bones beneath a skull, used to warn of danger or death, especially poison and piracy.

from The skull-and-crossbones image has deep roots in European memento mori art and church ossuaries, where bones were arranged this way to remind the living of death's certainty. It was later adopted by pirates — the Jolly Roger — possibly to borrow that same dread for intimidation, since a flag announcing 'we bring death' discouraged resistance before a shot was fired. Its use as a poison warning came later still, spreading in the 1800s and 1900s as a simple, literacy-free way to say 'this will kill you,' precisely because the symbol already carried centuries of that meaning.