UNREAL (5 letters) The term 'not rational' can be described by two crossword solutions: 'INSANE' (6 letters) and 'UNREAL' (5 letters). INSANE is a term used to describe someone who is mentally ill, often in a way that affects their behavior and ability to think clearly. UNREAL is a term used to describe something that is not real or not based in reality. Both terms can be used to describe something that is not rational.