clam up

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clam up

To suddenly stop talking or refuse to speak, typically due to a particular reason. The phrase refers to a clam closing its shell. Naturally, everyone clammed up as soon as the check came. Our usual informant clammed up when he realized there was a mole in the department.
See also: clam, up
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

clam up

to say nothing. (Closing one's mouth in the way that a clam closes up.) The minute they got him in for questioning, he clammed up. You'll clam up if you know what's good for you.
See also: clam, up
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.

clam up

Refuse to talk or respond, as in Whenever she asks her teenager about his activities, he clams up. This term alludes to the tightly closed valves of a live clam. [Slang; early 1900s]
See also: clam, up
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

clam up

v. Slang
To refuse to talk or to stop talking suddenly: The politician clammed up when the reporters started to ask about the scandal.
See also: clam, up
The American Heritage® Dictionary of Phrasal Verbs. Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

clam up

in. to get quiet. (see also dummy up.) The minute they got him inside the cop-shop, he clammed up.
See also: clam, up
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.
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