not on

be not on

To be inappropriate and/or unfair, as of one's behavior. I'm sorry, but it's just not on to spread gossip about someone.
See also: not, on

(just) not ˈon

(informal) not a good or an acceptable way to behave: He told her that using his bike without asking just wasn’t on.It’s not on — you should know better.
See also: not, on