noun.
An eccentric, insane, crazy or reckless person.
noun.
A person who is obsessed with something.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
noun.
a person who is regarded as eccentric or mad
Word Usage
"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."