be a no-no

be a no-no

To be objectionable or prohibited, as of an action. If you're trying to become friends with this woman, criticizing her style of parenting is a no-no. Talking in the middle of a Broadway play is a real no-no.
Farlex Dictionary of Idioms.
See also:
  • no-no
  • no-no, a
  • a no-no
  • nulla poena sine lege
  • sine
  • there ought to be a law
  • There ought to be a law!
  • there oughta be a law
  • oughtta
  • credit to