clam up phrasal verb (see also clam)
informal to suddenly stop talking, especially when you are nervous or shy:
A sensitive child is likely just to clam up.

[TahlilGaran] Dictionary of Contemporary English ▲

clam up
to refuse to talk or answer.
Every time I think he's going to tell me what's bothering him, he just clams up.

Etymology: based on the behavior of a clam (= a sea creature), which quickly closes its shell when something touches it

[TahlilGaran] English Idioms Dictionary ▲

clam up
to close one's mouth, to shut up; to stop talking, to refuse to talk (like a clam closes its shell)
The students clammed up when they saw the teacher.

I had to clam up during the meeting so my boss would not become angry with me.

The girl clammed up when her boyfriend entered the room.

[TahlilGaran] English Idioms Dictionary ▲

clam up
v., slang To refuse to say anything more; stop talking.
The suspect clammed up, and the police could get no more information out of him.

[TahlilGaran] English Idioms Dictionary ▲