The term 'It's a No-No' is often used to describe something that is not allowed or is considered wrong. The crossword solutions for this term are TABOO (5 letters) and SIN (3 letters). TABOO is a word that is used to describe something that is forbidden or not accepted by society, while SIN is a religious term that refers to an action that goes against the teachings of a particular faith. Both words can be used to describe something that is not allowed or is considered wrong.