hold your tongue
US /hoʊld jʊər tʌŋ/
UK /hoʊld jʊər tʌŋ/

1.
to remain silent and not speak, especially when you want to say something negative or critical
:
•
I had to hold my tongue when my boss made that ridiculous suggestion.
•
It's best to hold your tongue if you don't have anything nice to say.