Something you mustn't do (2-2)
I believe the answer is:
no-no
(Other definitions for no-no that I've seen before include "Non-starter" , "Impossibility" , "turn-off for some?" , "Unacceptable act" , "Something totally unacceptable (informal)" .)
I believe the answer is:
no-no
(Other definitions for no-no that I've seen before include "Non-starter" , "Impossibility" , "turn-off for some?" , "Unacceptable act" , "Something totally unacceptable (informal)" .)