- refusal to talk further or reply
- to not give out any information
- to close down when danger is sensed
- She clams up every time I walk in, it is worrisome.
- The thief clammed up when he was taken for interrogation by the police, they could not get any information from him.
- Her son clams up every time he feels guilty of something.
- He was talking about something but clammed up the minute his wife entered. Maybe they aren’t getting along that well anymore.
- They ought to share more things between the two of them, clamming up in a marriage is never a good sign.
- The human resources team member will come and speak with her now. She seems too shocked with the incident and has clammed up completely.
- Psychiatrists have to regularly deal with patients that have clammed up. It sometimes takes hours and days but they do not give up.
A speculation is that the clamming up or shutting down comes from actual clams that shut down when the see any danger approaching. The literary origin of the phrase is however, unavailable.