Something, typically an action, that is objectionable or prohibited. If you're trying to become friends with this woman, criticizing her style of parenting is a no-no. Talking in the middle of a Broadway play is a real no-no.
a ˈno-no(informal) a thing or a way of behaving that is not acceptable in a particular situation: Women wearing trousers used to be a bit of a no-no, but now it’s completely normal.
n. something that is not (to be) done. (Essentially juvenile.) She seems to delight in doing all the no-nos.