A word to the wise ain't necessary. It's the stupid ones who need the advice.

Cosby’s line riffs on the proverb “A word to the wise is sufficient,” flipping it into a comic observation about advice and receptiveness. The “wise” don’t require much instruction because they infer, learn quickly, or already grasp the point; the people who most need guidance are often the least prepared to take it. The humor comes from the blunt, colloquial phrasing (“ain’t,” “stupid ones”) and the cynical implication that advice is frequently wasted on its intended audience. As a standalone aphorism, it also comments on the limits of moralizing: insight can’t be forced, and counsel works best when the listener is already inclined to reflect.