no-no 
noun (plural no-nos) informal
A thing that is not possible or acceptable: perming highlighted hair used to be a definite no-no, but it’s now possible...
- So an office at that end of the building is a definite no-no as the droning hum could drive me mad.
- That sort of event was a definite no-no and he worked hard to prevent such things.
- Wiggling out was a definite no-no since he had already made sure that I had nowhere else to turn to.
