1. v. phr. , slang To say something of great importance or meaning; say more by a sentence than the words usually mean. Usually in past tense. Tom said a mouthful when he guessed that company was coming to visit. A dozen people came. 2. v. phr. , informal To vent one's honest opinion, even in anger. He sure said a mouthful when he told his boss what was wrong with our business.
Antonym: GET AN EARFUL.