Carl Friedrich Keil and Franz Delitzsch Old Testament Commentary

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.