C. F. Keil and Franz Delitzsch on Proverbs 18:20

Pro 18:20 20 Of the fruit which a man’s mouth bringeth is his heart satisfied;      By the revenue of his lips is he filled.

He will taste in rich measure of the consequences not merely of the good (Pro 12:14, cf. Pro 13:2), but of whatever he has spoken. This is an oxymoron like Mat 15:11, that not that which goeth into the mouth, but that which cometh out of it, defileth a man. As at Joh 4:34 the conduct of a man, so here his words are called his βρῶμα. Not merely the conduct (Pro 1:31; Isa 3:10), but also the words are fruit-bringing; and not only do others taste of the fruit of the words as of the actions of a man, whether they be good or bad, but above all he himself does so, both in this life and in that which is to come.