that is a mystery. No one knows for sure but here are the leading theories:
One of the mysteries of the game of baseball is the origin of the term bullpen, the name for the area in which relief pitchers warm up. Several competing theories, none with compelling evidence to support or refute them, vie for the origin. About all we know for sure is the earliest recorded use of the term to refer to the pitchers' warm-up area was not until 1915, in Baseball Magazine .
One theory is that around the turn of the century relievers would warm up near the outfield fence, where signs for Bull Durham Tobacco. The picture of the bull, associated with the pitchers, who were usually the largest and strongest members of the team, was enough to create the imagery for the term.
Another, perhaps somewhat more likely, theory comes not from the use of relievers, but rather from late-arriving fans. In the 1870s, and perhaps earlier, after the game had started tickets would be sold at a discount. These late arriving fans with cheap tickets would be herded into a roped-off, standing-room-only area in foul territory. Because the fans were herded in like cattle, the area was known as the bullpen. In 1877 the Cincinnati Enquirer used the term to describe this practice. The name for this area of foul territory stuck, and later when relief pitchers became part of the game, they used this area of foul territory to warm up. After that, the term bullpen stuck to relievers.
And yet another explanation, not even directly associated with baseball, may be the origin. The term bullpen was used as early as 1809 to refer to a stockade or jail. By 1903, O. Henry was using the term to refer to any waiting area. Perhaps the generic term for waiting area simply became a more specialized term when applied to the sport.
Almond suggests that the origin came because a pitcher who had been knocked out of the box had been "slaughtered," and that reliever was just another bull to be slaughtered too. This explanation seems to be a reach.
Finally, no less than Casey Stengel weighed in on the subject. Stengel claimed that it was called the bullpen because that is where pitchers would sit and shoot the bull. This is probably more indicative of Stengel's opinion of relief pitchers than of the term's origin.<br><br><span class=gensmall>Accepted as answer by Arlo on 15 Apr 2003 08:08 EST.</span>