big mouth
US /ˈbɪɡ ˌmaʊθ/
UK /ˈbɪɡ ˌmaʊθ/

1.
おしゃべり, 口が軽い人
a person who talks too much, especially about things that should be kept secret
:
•
Don't tell him your secret; he's a real big mouth.
彼に秘密を話さないで。彼は本当におしゃべりだから。
•
Her brother is such a big mouth, he always spills the beans.
彼女の弟は本当におしゃべりで、いつも秘密を漏らす。