Klein Dictionary, כסל

כסל to be sluggish, be stupid.
    — Qal - כָּסַל was stupid (a hapax legomenon in the Bible, occurring Jer. 10:8 in the form יִכְסָלוּ). [Arab. kasila (= was sluggish). The orig. meaning of this base prob. was ‘to be thick, be plump, be fat’. For sense development cp. base טפשׁ, which unites the meanings ‘to be gross, be fat, be dull, be stupid’.] Derivatives: כְּסִיל ᴵ, כֶּסֶל ᴵᴵ, כִּסְלָה, כִּסְלוּת, prob. also כֶּסֶת ᴵ.