A foolish or confused person; someone who lacks common sense or clear thinking.
Compound of 'addle' (confused) + 'head,' formed similarly to other insults like 'blockhead' or 'hothead.' It's an English formation emphasizing where the problem supposedly lies.
Old English insults loved compound words where you combined an insult with a body part—'addlehead' suggests that confusion lives literally inside someone's head, as if you could see it sloshing around.
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