What it means
It means doing something too early, before you've actually had the go-ahead, then looking a bit silly when reality taps you on the shoulder. It comes from racing, where a runner who sets off before the starter's gun has literally jumped the gun. In everyday life, it's for spilling news too soon, booking things before they're locked in, or getting ahead of yourself before the moment's real.
Usage examples
"I tweeted the launch date like it was final, then the client delayed it a month. I jumped the gun and had to apologise to everyone."
"Don't jump the gun and book the flights before the dates are confirmed."
"I know you're excited, but don't jump the gun and tell your mum we're moving till the lease is signed."
"He jumped the gun and bought celebration drinks before the exam results were even out."
"She jumped the gun and put congrats on his post before he'd even said he got the job."
Where it comes from
It comes from track and field. If a runner started before the starter's pistol fired, that was a false start, so they'd literally jumped the gun. From there it moved into everyday English and now gets used for anyone who acts, announces, or commits too early before the proper signal's there.
