Fig. to say a lot; to say something very important or meaningful. When you said things were busy around here, you said a mouthful. It is terribly busy. You sure said a mouthful, Bob. Things are really busy.
Utter something important or meaningful, as in You said a mouthful when you called him a fine musician. This term is often used to express agreement, much as you can say that again is. It was first recorded in 1790.