What does skull and cross bones on an anchor mean?

Danger or Poison.

The most common interpretation of the skull and crossbones symbol is that it is a warning of danger or death. This association likely originates from the fact that the symbol has been used for centuries by pirates to mark their ships and flags. Pirates would often fly the skull and crossbones to warn other ships to stay away, as they were not afraid to attack and plunder.

This symbol is also often associated with poison, as it was once used to mark bottles and containers that contained poisonous substances. This association is particularly strong in the medical field, where the skull and crossbones symbol is now universally recognized as a warning that a substance is toxic or dangerous.