Definition: To begin before everything is ready.
Example: Joe realised too late that he had jumped the gun trying out his newly assembled bike at the store. The mechanic hadn’t put the seat on yet. The store was very good about the whole situation though. They offered to call him an ambulance.
Exported around the English speaking world from the United States, jump the gun dates back to the 1830s. It has its roots in track and field sporting events, referring to somebody starting the race before the starter’s pistol has been fired. Blindingly obvious once pointed out, isn’t it? It was frequently quoted as ‘beat the gun’ as well, though that usage seems to have died out.