An eccentric, insane, crazy or reckless person.(noun)
A person who is obsessed with something.(noun)
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Use "nutter" in a sentence
"I think just saying Hitler was a nutter is an oversimplification - it makes the most sense to think of him as a talented amateur who tried to micromanage."
"And I would use the word "nutter" - not of Edwin, obviously."
"Like, the kind of nutters that make us wonder whether the spirit said nutter is channeling (no really, as in: "My book is the Memoir of Gwynyfynyyynn, Princess of the Lost Underwater Kingdom of Mu, As Dictated Through the Willing Medium of My Person") is TOTALLY WASTED (OMG that's kind of an awesome book idea, is it not?"