clam up

B2 informal intransitive

In simple words

To suddenly become very quiet and refuse to say anything, like you've closed your mouth tightly and won't open it

Literal meaning: A clam (shellfish) closes its shell very tightly when threatened — to 'clam up' is to close yourself off like a clam

Meanings

1 B2 idiomatic informal

common

To suddenly stop talking and refuse to speak or answer questions, especially due to nervousness, fear, or stubbornness

"Every time I try to ask him what's wrong, he just clams up and leaves the room."

2 B2 idiomatic informal

common

To refuse to give information to an authority (e.g. police), typically out of fear or a desire to protect others

"The witness clammed up as soon as the detective started asking about the other suspects."

Usage notes

The image is of a clam (a shellfish) closing its shell tightly — just as a clam cannot be opened easily, a person who has 'clammed up' refuses to be drawn out. Commonly used when someone is questioned by police, parents, or in an interview and refuses to speak. Not used in formal writing.

Commonly used with

suddenly completely questioned interview police nervous

Forms

Base

clam up

I/you/we/they

3rd person

clams up

he/she/it

Past simple

clamed up

yesterday

Past participle

clamed up

have + pp

-ing form

claming up

continuous

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