shoot your mouth off

US /ʃuːt jʊər maʊθ ɔːf/
UK /ʃuːt jʊər maʊθ ɔːf/
"shoot your mouth off" picture
1.

口が軽い, 口を滑らせる

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

:
He tends to shoot his mouth off after a few drinks.
彼は数杯飲むと口が軽くなる傾向がある。
Don't shoot your mouth off about the surprise party.
サプライズパーティーについて口を滑らせるな