clam up
phrasal verb with clam verb uk/klæm/ us/klæm/ -mm-
informal (also shut up like a clam)
（通常指因为尴尬、紧张或不愿谈论某一话题而突然）沉默不语，闭口不言，拒不开口
to become silent suddenly, usually because you are embarrassed or nervous, or do not want to talk about a particular subject
He just clams up if you ask him about his childhood. 要是你问起他的童年，他就干脆不说话了。