The term 'MAJOR NO-NO' can be broken down into two words, 'MAJOR' and 'NO-NO'. The crossword solutions for these words are 'SIN' (3 letters) and 'TABOO' (5 letters). 'SIN' is a religious term used to describe an action that goes against the teachings of a particular faith. It is often seen as a moral wrong or transgression. 'TABOO' is a social or cultural prohibition or ban on certain activities or behaviors. It is considered unacceptable or forbidden by society. Together, these two words form the phrase 'MAJOR NO-NO', which is used to describe something that is strongly discouraged or forbidden.