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Phrase(s): I must say good night.
an expression announcing the speaker’s intention of leaving for the night. • Jane: It’s late. I must say good night. Bob: Can I see you again? Jane: Call me. Good night, Bob. Bob: Good night, Jane. • Sue: I must say good night. Mary: Good night, then. See you tomorrow.
McGrawhill's American Idioms And Phrasal Verbs