Jump the Gun

Comes from track and field athletics, where a starter pistol signals the beginning of a race, and an over-eager runner who bursts forward at the slightest sound before the shot is fired is disqualified for a false start. The phrase was originally "beat the gun," with "jump" replacing "beat" as the figurative, non-athletic use spread. The earliest confirmed citation, from an Iowa farming publication in 1921, is already figurative rather than describing an actual race, suggesting the athletic image had already become common shorthand by that point.