Proverbs 18:4

The words of a man's mouth are as deep waters, and the wellspring of wisdom as a flowing brook.

The words of a man's mouth (are as) deep waters - i:e., a wise man's words, as the parallel clause shows. The Hebrew is 'iysh (H376), a good man; not 'aadaam (H120), the general term for man. His words are deep: not merely on the surface for display, like the fool's (Proverbs 18:1).

The well-spring of wisdom - inexhaustible, ever-flowing, not soon run dry, like the fool's shallow knowledge. There is a depth in it not easily fathomed.