Informal 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).
Informal A social blunder; a faux pas.
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.