BBB Etymology - Ballpark Estimate

In the days when all baseball games were played in the open air during daylight hours, newsmen would have liked to know precisely how many fans showed up for a given game. But they seldom found out because owners and managers were cagey and it was hard to get a precise headcount. Besides, publicity about a low turnout might keep people away from the next game. It became a standard practice to give a very broad estimate - plus or minus a few hundred or thousand - when asked about the size of the gate on a particular day.