Thing you mustn't do (4)

I believe the answer is:

nono

(Other definitions for nono that I've seen before include "Something totally unacceptable (informal)" , "Non-starter (colloq.)" , "Impossibility" , "Something not acceptaable" , "Failure (colloq.)" .)

I've seen this clue in the USA Today.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.