ˈno-no noun [singular] informal something that you must not do because it is considered to be unacceptable behaviour:
Colouring your hair was a no-no at that time. [TahlilGaran] Dictionary of Contemporary English ▲
be a no noif something that someone does is a no no, people do not think it is an acceptable way of behaving
[TahlilGaran] English Idioms Dictionary ▲