From kacal; properly, fat, i.e. (figuratively) stupid or silly -- fool(-ish).
see HEBREW kacal
I. כְּסִיל70 noun masculine stupid fellow, dullard, fool; — ׳כ Psalm 49:11 44t.; plural כְּסִילִים 94:8 25t.; — "" בער 49:11; 92:7; 94:8, elsewhere only in Wisd Lt.; he hates knowledge Proverbs 1:22; delights not in understanding 18:2; it is his sport to do mischief 10:23; his heart proclaimeth אִוֶּלָת 12:23; his mouth poureth it forth 15:2, and feedeth on it 15:14.