noun.
Lack of reason or rationality; unreasonableness; irrationality.
noun.Nonsense; folly; absurdity.
verb.
To prove to be unreasonable; disprove by argument.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
noun.
the state of being irrational; lacking powers of understanding
Word Usage
"Or again, the term unreason is sometimes used rather more reasonably; for a sort of loose or elliptical statement, which is at least illogical in form."