Taboo thing (4)

I believe the answer is:

nono

(Other definitions for nono that I've seen before include "Unacceptable thing" , "Something that must not be done (informal)" , "Italian composer" , "Something not acceptaable" , "Failure (colloq.)" .)

I've seen this clue in the King Feature Syndicate, the Universal, The Wall Street Journal and the Newsday.

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