An ass pirate is a slang term to describe penetration into a partner's anal cavity. It is implied that the individual penetrating exhibits pirate-like behavior in pillaging the anus (having rough anal sex) or by searching for treasures in the anus by inserting a penis or anal toy inside the anal cavity and prodding around.
More About Ass Pirate
The term ass pirate projects a pirate's behavior into anal sex. Pirates are often known for pillaging ships, rough and rowdy behavior, and the search for hidden gold treasures. An ass pirate is someone who takes on similar actions during anal sex by pillaging the anus during rough anal sex or viewing the anus as the equivalent of a hidden cave containing treasures to be found.