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.
A prohibition or taboo; something that is forbidden or at least discouraged. The term dates from the mid-1900s and was popularized in the late 1960s in a television show, Rowan & Martin’s Laugh-In. Danielle and John Kinkel used it in the Boston Globe (Feb. 24, 2005) in a piece about the shortage of Roman Catholic priests: “First, everyone knows that ordaining women is a papal no-no and will not be acted upon in the near future.”