This expression for an educated guess or a close estimate comes from the American ball fields.. . In the old days, baseball games were only played during daylight hours. News reporters covering the games wanted to know the attendance so they could report it in the evening paper. Managers would never want to reveal a low figure, so they guessed - giving a broad or ballpark figure.. . But where did "ballpark" come from? Baseball stadiums were called ballparks beginning in the early 1900s. Before that they were ball grounds and ball fields. "Baseball stadium" may be popular usage today, but "ballpark" prevails in expressions like "in the ballpark," meaning well within reach.