Help yourself.; After you. (A polite way of indicating that someone else should go first, help himself or herself to something, or take the last one of something.) •
Mary: I would just love to have some more cake, but there is only one piece left. Sally: Be my guest. Mary: Wow! Thanks!
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Jane: Here's the door. Who should go in first? Bill: Be my guest. I'll wait out here. Jane: You're so polite!