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

1.
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.
•
Her brother is such a big mouth, he always spills the beans.
a person who talks too much, especially about things that should be kept secret