no-no (n??n??)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.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.no?-no` n., pl. -nos, -no’s. Informal. anything that is forbidden or not advisable, as because of being improper or unsafe. [1940?45] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.