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no-no

[noh-noh]
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noun, plural no-nos, no-no's. Informal.
  1. 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.
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Origin of no-no

1940–45, Americanism; reduplication of no1
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Word Origin and History for no-nos

no-no

n.

1942, from reduplication of no.

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Online Etymology Dictionary, © 2010 Douglas Harper