Cat got your tongue?

Cat got your tongue? Has the cat got your tongue? The cat's got my tongue. The cat has my tongue.

This common idiom refers to a situation when you cannot think of the word or name you want, even though you know it very well.

"He's at..um...oh, I can't remember the name of it! The cat has my tongue. I'll think of it in a second. I go there all the time."

It can also be used to refer to a general inabilty to express thoughts clearly.

It is also used rhetorically when someone is expected to speak, reply, or respond, but does not, or when the response is started, but is a choppy, stumbling response that never finishes. ("It's a very- I think- well he never- well, I mean, if you- I don't really-") This person has started several sentences but can't seem to decide what to say.

What's the matter? Cat got your tongue?

This rhetorical usage can be a friendly joke, or very confrontational, so use it carefully. Tone of voice, the speaker's relationship with the other person, and the situation can help make the meaning more clear.