bullpen (n.)

also bull-pen, 1820, "pen or enclosure for bulls," from bull (n.1) + pen (n.2). Baseball sense "area where pitchers warm up before entering a game" is from 1915, perhaps from earlier slang meaning "temporary holding cell for prisoners" (common in American Civil War camps). Bullpen also was the name of a baseball-like game played in U.S. late 19c.