Proverbs 23:9

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 (Psa 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 Pro 6:30.

We are here directed not to cast pearls before swine (Mat 7:6) and not to expose things sacred to the contempt and ridicule of profane scoffers. It is our duty to take all fit occasions to speak of divine things; but, 1. There are some that will make a jest of every thing, though it be ever so prudently and pertinently spoken, that will not only despise a wise man's words, but despise even the wisdom of them, that in them which is most improvable for their own edification; they will particularly reproach that, as if it had an ill design upon them, which they must guard against. 2. Those that do so forfeit the benefit of good advice and instruction, and a wise man is not only allowed, but advised, not to speak in the ears of such fools; let them be foolish still, and let not precious breath be thrown away upon them. If what a wise man says in his wisdom will not be heard, let him hold his peace, and try whether the wisdom of that will be regarded.

23:9 Saying 9: Fools ignore advice (cp. 9:7-12; 10:18), so the wise do not waste their breath (see Matt 7:6).