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n. pl. no-noes Informal
1. Something unacceptable or impermissible: "Even though his company wasn't the one involved in the case, what he did is considered a definite no-no" (Mike Royko).
2. A social blunder; a faux pas.
3. Baseball A game in which a pitcher allows no hits and no runs.
n., pl. -nos, -no's.
Informal. anything that is forbidden or not advisable, as because of being improper or unsafe.