From October 28, 2012:
That saying seems to have first pieced together in the Darwin era, when it was found that monkeys are our genetic cousins. When they found out that apes and monkeys would get intoxicated eating fermented bananas, kid-friendly cartoons started to show monkeys and apes going nuts when they saw bananas. So with humans beings their cousins, and the cartoons showing apes and monkeys going crazy, the idiom was coined that a person going crazy would be going bananas. This may or may not have spurred the idea that heating and ingesting the peel of the fruit would result in hallucinations, but the alcohol content in fermented bananas is significant enough that there's a drink in Indonesia made from it.
This is just my piecing together from a couple of sources. The idioms true origin is shrouded in mystery, some thinking that it was started on college campuses in the 60s. But as far as I do know, the usage of the word "bananas" means someone is crazy, mostly like being drunk crazy, which hosts a number of reasons for how the idiom appeared during the late 60s or even the early 70s.
- Anya
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