What is the idiomatic meaning of clam up?

The idiomatic meaning of "clam up" is to become silent or refuse to speak, especially when questioned. It implies a deliberate shutting down of communication, often due to nervousness, embarrassment, or a desire to conceal information.

Here are some examples:

* "When the police arrived, the suspect clammed up and refused to answer any questions."

* "She was so embarrassed by her mistake that she just clammed up."

* "He usually talks a lot, but he clammed up when I asked him about his weekend."

The idiom originates from the image of a clam closing its shell, symbolizing a closed-off, silent state.