clam up

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

闭口不言, 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.
当警察开始提问时,他只是闭口不言
She tends to clam up when she's nervous.
她紧张时往往会闭口不言