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1.
Someone who suffers from a mental disorder and delusions of grandeur. Someone who has lost touch with reality. Could be just depressed but rans the gammit from depression to schizophrenia.
Man, that Mike Rehmeyer is mentally ill. He's 34, lives at home, has no college education and nothing going for him yet he thinks he's Prince Charles.
by arfed February 12, 2008
 
2.
See atheist.
Jack the atheist was mentally ill.
by Skialian January 09, 2014