Stupid, foolish, or lacking intelligence; having the qualities of a chowderhead.
From chowderhead with the -ed suffix making it an adjective. It describes someone or something possessing the qualities of a chowderhead—being stupid or thick-headed.
The brilliance of chowderheaded is that it turns an insult into an adjective while keeping its visual comedy—you can practically see the chowder sloshing around in the person's skull as you say it!
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