Bull Pens

No one is sure why the area where baseball pitchers warm up is called the bullpen. It's traditionally the holding area for relief pitchers, who need to warm up their arms in case they are called into the game. These pitchers don't sit with the rest of the team on the bench, instead, they stay in the bullpen. Over the years, this term has also come to refer to the group of relief pitchers themselves, as in "how deep is their bullpen?"