I must be off
I have to leave. A somewhat formal and dated expression. Adam's waiting for me, so I must be off. Ooh, the train comes in five minutes, so I must be off!
I must be off.
an expression announcing the speaker's intention of leaving. Bill: It's late. I must be off. Bob: Me, too. I'm out of here. Sue: I must be off. John: The game's not over yet. Sue: I've seen enough.