1
general::
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.
American Heritage Idioms
2
general::
Phrase(s): say a mouthful
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.
McGrawhill's American Idioms And Phrasal Verbs