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How did “nuts” or “bananas” come to mean “crazy”?
The English language has long had a flippant attitude toward mental illness. Any sort of abnormal behavior can get you called crazy. And if you actually are crazy, in other words, mentally ill, it’s not very nice when people call you crazy. That’s created a lot of light-hearted synonyms for crazy or weird or not quite right. Here’s how a few of them developed.
By the middle of the 19th century, it was common slang to refer to one’s head as a nut. It makes sense—the head is round, like a nut, and the important stuff is all encased in a thick, protective shell (your skull). Etymologists say that around 1860, the phrase “to be off one’s nut” came into common usage, and it meant insane—if you were off your nut, you were out of your head. (Similarly, nutter is British slang for an insane person). But “off one’s nut” didn’t become the much shorter, simpler “nuts” until the 1930s, thereby dropping all of the head-and-nut imagery.
This one comes from a long series of corruptions and adaptations of slang use of the word “bent.” In the 1910s, bent was criminal slang that meant “crooked” (used by criminals who thought other criminals were crooked). By the 1930s, it had evolved to mean illegal, in general. From there, somebody started saying “bananas” instead of bent, because bananas are crooked. It’s a quick step from crooked to just plane crazy, and that’s what happened when the word was used in the popular 1930s crime slang book The Underworld Speaks. It was published by the FBI to help average people spot gangsters by their slang.