I'm guessing you mean "clam up," as Michael Wheeler says. It's a familiar U.S. colloquial expression, but, also as Wheeler says, I'm actually sure I've heard it spoken in real life. To me it carries a sense of "to stop talking BECAUSE you are about to reveal a secret."
Google News finds me some examples:
"doctors treating illegals are being told to clam up" (keep it secret)
"Politicians must choose words carefully to protect society... Perhaps the best way is to just clam up and avoid gaffes altogether." (A "gaffe" is an embarrassing mistake).
"How to talk about money before you marry... Your future mate will most likely clam up the second they feel they are being interrogated."