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).
n. something that is not (to be) done. (Essentially juvenile.) : She seems to delight in doing all the no-nos.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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[noh-noh] 
