Be taking medication or an illegal drug, as in Are you on some antibiotic? or He was definitely on narcotics when it happened. [1930s]
Be in favor of something or willing to participate, as in We're going dancing after the play—are you on? [Colloquial; late 1800s]
Be engaged in some action, especially on the stage, as in Hurry up, you're on in five minutes. [Late 1700s]
Perform extremely well, as in I can't return Dan's serve—he's really on today. [Slang; second half of 1900s]
Be scheduled, as in Is tonight's rally still on? [Colloquial; second half of 1990s]
be on one. Be at one's expense, either as a treat or the butt of a joke. For example, This round of drinks is on me, or He enjoys a good laugh, even when the joke's on him. [Colloquial; second half of 1800s]
not be on. Be unacceptable, not allowable, as in I can't believe you'd cancel; that's just not on. This usage is more common in Britain than America. [Colloquial; 1930s] For a synonym, see not done. Also see be on to.