The "fool" here is one willfully and persistently deaf to it, almost identical with the scorner.

Speak not in the ears of a fool,.... For it is only beating the air, and speaking to the wind; it is casting pearls before swine, and that which is holy to dogs. By the "fool" is meant a wicked man, one abandoned to sin, and hardened in it; that scoffs at all admonitions and reproofs, that derides the word, and the preachers of it, and makes a mock at all good men, and everything they say; and therefore what is serious and sacred should not be said to them, since it only becomes the object of their banter and ridicule;
for he will despise the wisdom of thy words; not only the words of doctrine, reproof, and correction, but the "wisdom" of them; or let them be ever so wisely spoken; for if the wisdom of God and his words, the truths of the Gospel, are foolishness with such, and despised by them, then much more the wisdom even of the best of men, and the wisest things they say; yea, when they deliver the wisdom of God in a mystery, the hidden wisdom, the Gospel of Christ, which therefore should be spoken among them that are perfect, 1-Corinthians 1:24.

It is our duty to take all fit occasions to speak of Divine things; but if what a wise man says will not be heard, let him hold his peace.

(Compare Proverbs 9:8). "Cast not your pearls before swine" (Matthew 7:6).

Another case in which good words are lost:
Speak not to the ears of a fool,
For he will despise the wisdom of thy words.
To speak in the ears of any one, does not mean to whisper to him, to so to speak that it is distinctly perceived. כּסיל, as we have no often explained, is the intellectually heavy and dull, like pinguis and tardus; Arab. balyd, clumsy, intellectually immoveable (cf. bld, the place where one places himself firmly down, which one makes his point of gravity). The heart of such an one is covered over (Psalm 119:70), as with grease, against all impressions of better knowledge; he has for the knowledge which the words spoken design to impart to him, no susceptibility, no mind, but only contempt. The construction בּוּז ל has been frequently met with from Proverbs 6:30.

Of a fool - Cast not away good counsels upon incorrigible sinners.