Crazy or insane people (informal/slang); or people who collect or harvest nuts.
From nutter (one who gathers nuts, or slang for a crazy person), from nut plus -er. The slang meaning emerged in British English in the early 20th century, possibly from 'soft in the head like a nutshell.'
British slang calls crazy people 'nutters' possibly because a cracked nutshell suggests a cracked mind—but the same word can mean someone who harvests nuts, showing how one root word got two completely opposite meanings in different communities.
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