Since they hail from a city situated on three rivers, you'd think that the Pirates' name is somehow related. But no: In fact, it hails from a scandal perpetrated in 1890. The team—then roughly 15 years old, having gone through a multitude of reshufflings and renamings—was scouting a player named Lou Bierbauer who had played for the Philadelphia Athletics. Bierbauer was due to return to Philly after a disastrous season in Pittsburgh, but according to Philly's managers, Bierbauer was accidentally left off of their roster. The Pittsburgh team (then known as the Alleghenys) snatched him up anyways. And in a letter condemning their actions, a league official called the team's act "piratical." Pittsburgh embraced it.
It didn't hurt that the Pirates were known for their swashbuckling antics. For example, in 1887, a particularly bad season was capped off by the burial of one player's pet monkey beneath home plate in what Pittsburgh City Paper writer Chris Potter calls "moving pre-game ceremony."