no-no

[ noh-noh ]
/ ˈnoʊˌnoʊ /

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

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