The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with say a mouthful
say a mouthful
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.