Be taking medication or an illegal drug, as in Are you on some antibiotic? or He was definitely on narcotics when it happened. [ 1930s ]
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Be in favor of something or willing to participate, as in We're going dancing after the play—are you on? [ ; late 1800s ]
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Be engaged in some action, especially on the stage, as in Hurry up, you're on in five minutes. [ Late 1700s ]
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Perform extremely well, as in I can't return Dan's serve—he's really on today. [ ; second half of 1900s ]
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Be scheduled, as in Is tonight's rally still on? [ ; second half of 1990s ]
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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. [ ; second half of 1800s ]
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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. [ ; 1930s ]