A silly or stupid person; someone who makes foolish mistakes or doesn't think clearly.
A compound word combining 'fat' (thick) and 'head,' first appearing in American English around the 1920s. It's an insult comparing thick skull with lack of intelligence, though there's no actual connection between head size and intelligence.
This is a great example of how insults don't need to make logical sense—we still use 'fat,' 'thick,' and 'dense' to mean 'stupid' even though brain size has nothing to do with intelligence!
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