From Longman Dictionary of Contemporary Englishno-noˈno-no noun [singular] informal NOT DO somethingsomething that you must not do because it is considered to be unacceptable behaviour Colouring your hair was a no-no at that time.
Examples from the Corpus
no-no• Earrings and heavy chains around the neck are a no-no on the field.• Handwritten signs in the restaurant are a no-no.• The use of fresh ground beef is a no-no.• Apparently, this is a no-no in insurance land.• In technical terms, this is a big no-no.• And crying for me was a big no-no in business life.• A word of warning, though-counters are generally considered a design no-no.• There is lots of touching, and I think touching, in our society, is a real no-no. 