In a stupid or thoughtless manner; acting without intelligence or care.
Adverbial form of 'fatheaded,' created by adding the -ly suffix to the adjective. This follows standard English word formation patterns where adjectives describing mental states become adverbs describing actions.
The fact that we needed a specific adverb for this insult shows how vivid 19th-century American slang was—they didn't just call someone stupid, they turned it into a whole family of words describing stupidity in action.
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