Where did the term “bullpen” come from?

The origin of the term “bullpen” has long been debated. Casey Stengel claimed that managers, sick of their extra pitchers just sitting around “shooting the bull,” would send them to pens in the outfield that resembled pens used for bulls or cows. Johnny Murphy believed the name came from Bull Durham advertising signs located near the bullpens in many the ballparks. The term “bullpen” had also been used to name the place where inmates exercise in prison. A final idea is that relief pitchers are like reserve bulls in bullfighting, penned on the side and called into action if the starting bull is inadequate.