the cat has got someone's tongue
the cat's got (one's) tongue
One is unable or unwilling to speak. Used especially in the form of the question, "(has the) cat got your tongue?" I tried asking him about the incident, but it seems the cat's got his tongue. A: "What's wrong, sonny? Cat got your tongue?" B: "Oh, he's just shy around new people, that's all."
See also: tongue