Jump the gun
verb
To do something too soon, especially without thinking carefully about it.
He jumped the gun and sent the email without proofreading it first.
Often appears as...
jump the gun on
jumped the gun by
Usage tips
Informal, DisapprovingIdiomatic
Informal, DisapprovingIdiomatic
Definition 1 of 2
Pro Tip 1/3Impulsiveness Caution
The phrase often implies a lack of thought or planning, suggesting impulsiveness. 
He jumped the gun on the project and now we have to start over.
Pro Tip 2/3Negative Outcome
Using 'jump the gun' usually points to a negative result due to early action. 
Jumping the gun led to a misunderstanding with the client.
Pro Tip 3/3Metaphorical Use
Despite its origin in sports, the phrase is often used metaphorically in different situations. 
Don't jump the gun and assume they will agree to the deal.