clam up

US /klæm ʌp/
UK /klæm ʌp/
"clam up" picture
1.

閉口不言, 守口如瓶

to suddenly stop talking, especially when you are expected to speak

:
When the police started asking questions, he just clammed up.
當警察開始提問時,他只是閉口不言
She tends to clam up when she's nervous.
她緊張時往往會閉口不言