English Idioms and Phrases: Jump the gun
If you jump the gun you are starting to do something before you are fully prepared, so for example you are starting to build a house before you have planned what it will look like.
"You haven't planned the route you will drive, don't you think you are jumping the gun by leaving now"
"You should ask her to move in with you before you buy the house, you don't want to jump the gun"
In a track and field race like the 100 metre sprint the start of the race usually began a pistol being fired. Someone would shout "on your marks, get set" and then fire the gun into the air to signify the beginning.
Some athletes however would begin to run before the gun was fired and therefore would literally jump the gun, so they would start running before the race has started. This is now known as a false start in athletics.
This phrase may also be used as 'beat the gun' or 'beat the pistol'
Alternatives or synonyms