Ballpark Figure

Comes from baseball, though not quite how most people assume. The Oxford English Dictionary traces the figurative phrase "in the ballpark" to 1943, as United States Air Force slang for a general, inexact geographical area, well before it had anything to do with numbers. According to lexicographer Stuart Flexner, the idea then shifted to estimates: being "in the ballpark" meant landing somewhere in a reasonable range, much like a batted ball that's hit too far to be exact but still lands inside the stadium. The specific phrase "ballpark figure" followed later, first recorded in the Seattle Times in 1968.