A primitive root; properly, to be fat, i.e. (figuratively) silly -- be foolish.
Englishman's Concordance
1) (Qal) to be foolish, be stupid Part of Speech: verb A Related Word by BDB/Strong’s Number: a primitive root Same Word by TWOT Number: 1011
Brown-Driver-Briggs
[כָסַל] verbbe or become stupid (Late Hebrew Aramaic in derivatives; Arabic be sluggish, so North Syriac in derivatives; original meaning possibly thick, plump, fat; hence in good sense: כְּסָלִיםloins, כֶּסֶל, כִּסְלָהconfidence; in bad sense: כֶּסֶל, כִּסְלָה, כְּסִילוּתstupidity, folly, כְּסִילstupid fellow) —
QalImperfectיִכְסָ֑לוּthey become stupidJeremiah 10:8 ("" יִבְעֲרוּthey become brutish).
Jeremiah 10:8 HEB:וּבְאַחַ֖ת יִבְעֲר֣וּ וְיִכְסָ֑לוּ מוּסַ֥ר הֲבָלִ֖ים NAS: stupid and foolish [In their] discipline KJV: brutish and foolish: the stock INT: are altogether stupid and foolish discipline of delusion