noun, plural no-nos, no-no's. Informal.
anything that is forbidden or not advisable, as because of being improper or unsafe: If you want to lose weight, rich desserts are a no-no.
Origin of no-no
1940–45, Americanism; reduplication of no1
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Related Words for no-nosobscenity, embargo, restriction, proscription, exclusion, constraint, veto, injunction, taboo, prevention, statute, decree, code, principle, law, order, procedure, control, restraint, stricture
1942, from reduplication of no.
Online Etymology Dictionary, © 2010 Douglas Harper