A primitive root; properly, to be fat, i.e. (figuratively) silly -- be foolish.
H3688. kasal
[כָסַל] verb be 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: כְּסָלִים loins, כִּסְלָה כֶּסֶל, confidence; in bad sense: כְּסִילוּת כִּסְלָה, כֶּסֶל, stupidity, folly, כְּסִיל stupid fellow) —
Qal Imperfect יִכְסָ֑לוּ they become stupid Jeremiah 10:8 ("" יִבְעֲרוּ they become brutish).