speak off the cuff
To say something casually and spontaneously, without planning or preparation. (Hyphenated if used as an adjective before a noun.) I didn't have time to organize my thoughts, so I just spoke off the cuff. You could tell she had been speaking off the cuff, because she couldn't properly answer the questions reporters asked her at the end of the conference.
Fig. to speak without preparing a speech; to speak extemporaneously; to render a spoken opinion or estimate. (As if one's notes had been written hastily on one's cuff.) she is capable of making sense and being convincing even when she speaks off—the-cuff. I find it very difficult to speak off-the-cuff.
See also: speak