(Has the) cat got your tongue?
A humorous question directed at one who is not speaking very much or at all. A: "What's wrong, sonny? Cat got your tongue?" B: "Oh, he's just shy around new people, that's all."
cat got one's tongue
A comment made when someone is unaccountably or unusually quiet, as in We haven't heard from you all morning-has the cat got your tongue? Often put as a question, this term originally was used mainly with a child who did something wrong and refused to answer any questions. Today it is used more generally to ask anyone to speak. [Mid-1800s]