[/nq] Because when one person uses "bananas" to mean "mad," other people understand what is meant. " MW online confirms this. " I would find this etymology much more plausible if some dictionary actually said that one definition of the noun "banana" was "worthless or crazy person," but I've never seen such a definition, not even in AHD; a search of dictionaries online (and my hard-copy MW3) turned none up; and I am unfamiliar with the supposed usage.
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