run off at the mouth

US /rʌn ɔf æt ðə maʊθ/
UK /rʌn ɔf æt ðə maʊθ/
"run off at the mouth" picture
1.

to talk too much, especially in a boastful or indiscreet way

:
He tends to run off at the mouth when he's had a few drinks.
Don't tell him your secrets; he'll just run off at the mouth to everyone.