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