Definition:
To "clam up" means to suddenly become silent or refuse to talk about something, often due to fear, shyness, or secrecy.
Use case:
You can use this slang when someone stops talking or becomes tight-lipped about a topic. For example, if you ask your friend about their new job, and they suddenly become quiet and refuse to answer, you could say "Why did you clam up? Is everything okay?" It can also be used in situations where someone is being interrogated by the police or in court and refuses to answer questions, they may be said to "clam up."
• I tend to clam up in big crowds because I get nervous.
• When my boss walked in, everyone clammed up and got back to work.
• Don't clam up on me now, I need to know what happened last night.
• Whenever I ask about her past, she just clams up and refuses to talk about it.
• My little sister always clams up when she's in trouble and won't tell me what she did.
• He knew he had to clam up and keep his mouth shut in order to protect his friends.