A word to the wise?

Solution 1:

As John Lawler says, the full form of the phrase is a word to the wise is sufficient. A word to the wise is usually employed as a discreet warning in which the ‘word’ is very brief—either the utterance which immediately follows or this utterance itself.

A word to the wise—Paul knows.
A word to the wise—

The sense is “I need say no more than a word to alert you—you’re smart enough to understand what I leave unsaid.”

Solution 2:

It implies in a backwards way:

If you're wise you'll listen to these words.

Solution 3:

Proverbs 17:10 A rebuke strikes deeper into a discerning person than a hundred blows into a fool.

In other words, the wise learn quickly and from few words as in "A word to the wise"

A fool on the other hand cannot learn no matter how many words or even blows.