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Phrase(s): Don’t hold your breath.
Fig. Do not stop breathing waiting for something to happen that won’t happen. (Meaning that it will take longer for it to happen than you can possibly hold your breath.) • Tom: The front yard is such a mess. Bob: Bill’s supposed to rake the leaves. Tom: Don’t hold your breath. He never does his share of the work. • Sally: Someone said that gasoline prices would go down. Bob: Oh, yeah? Don’t hold your breath.
McGrawhill's American Idioms And Phrasal Verbs