clam up

Example: “When the police asked where he'd been, he clammed up.”

Origin: Rests on a fact about clams that anyone who has tried to open one already knows: the adductor muscle holding the two halves of the shell together is extraordinarily strong for the animal's size, and a live clam cannot be pried open by hand at all — it requires a tool, and it will not tire or relent before the person does. That combination of complete closure and refusal to yield to pressure is exactly what the idiom describes, which is why it means falling silent under questioning rather than merely being quiet. The phrase is American slang, attested from the early twentieth century, and it belongs to a small cluster of American expressions built on the same shellfish, including "happy as a clam" — originally the fuller "happy as a clam at high tide," when the diggers cannot reach it.