The Answer
The name “pirate” refers to the team’s early behaviour of “pirating” players away from other teams.
Pittsburgh is nearly 400 miles from the nearest ocean. It seems unlikely that pirates would have played a big part in the city’s history.
At first, I had assumed that, like the LA Dodgers, the name was imported from another place when the team moved. That’s not the case with Pittsburgh.
After a decade or so of independent professional baseball in Pittsburgh, a team called the Alleghenys joined the relatively new National League in 1877. In 1890 a competing league called the Players League formed a team called the Pittsburgh Burghers. Most of the Alleghenys players left to join that team leaving that team severely handicapped, losing 113 games.
When the Players League folded after only one season, all players were expected to return to the clubs they played for in 1889. But in 1891 Pittsburgh signed Louis Bierbauer, a second baseman with Brooklyn of the Players League who had spent the previous four seasons with the Philadelphia Athletics. The A’s and their American Association brethren immediately accused Pittsburgh of “pirating” Bierbauer away from them. Pittsburgh kept the player and the nickname and became known as the Pirates thereafter.
The Pirates have gone on to win 5 World Series championships, most recently in 1979.