Matthew Henry Commentary Proverbs 26:6-9

"He that sendeth a message by the hand of a fool Cutteth off [his own] feet, [and] drinketh in damage. The legs of the lame hang loose: So is a parable in the mouth of fools. As one that bindeth a stone in a sling, So is he that giveth honor to a fool. [As] a thorn that goeth up into the hand of a drunkard, So is a parable in the mouth of fools." — Proverbs 26:6-9 (ASV)

Fools are not fit to be trusted, nor to have any honor. Wise sayings, as a foolish man delivers and applies them, lose their usefulness.