Source : Webster's Revised Unabridged Dictionary (1913)

   {Good form} or {Bad form}, the general appearance, condition
      or action, originally of horses, atterwards of persons;
      as, the members of a boat crew are said to be in good form
      when they pull together uniformly. The phrases are further
      used colloquially in description of conduct or manners in
      society; as, it is not good form to smoke in the presence
      of a lady.

Source : WordNet®

good form
     n : behavior that conforms to social conventions of the time;
         "it is not good form to brag about winning"