expression, godan verb (-ru)
clam up; keep one's mouth shut; say nothing
Idiomatic expression literally meaning 'to become a shellfish', used when someone suddenly stops talking or refuses to speak, often in a stubborn or guarded way.
彼は急に貝になって、何も話さなくなった。
He suddenly clammed up and stopped saying anything.
That kid clams up right away when he gets angry.