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.
Words nearby be on
Other Idioms and Phrases with be on (2 of 2)
see be on.
How to use be on in a sentence
"Very well," said Maud in a your-blood-be-on-your-own-head voice.Wanted: A Husband|Samuel Hopkins Adams