Good/bad form - Idioms by The Free Dictionary
Behavior or actions that are considered socially inappropriate, distasteful, or rude. Many people still consider it bad form to ask a woman her age.
good/bad ˈform (old-fashioned, British English) a way of doing things that is socially acceptable/not socially acceptable: I think it was very bad form for Joe to arrive late for the funeral. ♢ Apparently, good form requires you to wear a hat on these occasions.