clam up
Upper Intermediate (B2) informal
To suddenly become very quiet and refuse to say anything, like you've closed your mouth tightly and won't open it
"Every time I try to ask him what's wrong, he just clams up and leaves the room."
Hear it in context
5 Phrases You Must Use to Sound Confident & Undisturbed During Conflict | Jefferson Fisher
Jefferson Fisher · 77:51
to attack when that just makes them clam up more.
Joe Rogan Can't Make Up His Mind...
Mike Nellis · 10:10
But then they clam up and refuse to criticize this president