Taboo behavior (4)

I believe the answer is:

nono

(Other definitions for nono that I've seen before include "Something that must not be done (informal)" , "Something that would be totally unacceptable" , "Something totally unacceptable (informal)" , "Something not acceptaable" , "Unacceptable thing" .)

I've seen this clue in the Universal and the USA Today.

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