don't hold your breath

Don't expect something to happen. (The idea being that one couldn't hold one's breath long enough for the unlikely thing to happen.) I know you hope Monica comes to the meeting, but don't hold your breath—she hasn't been to one all year. A: "I hope they get to my part of the performance today." B: "Don't hold your breath—you're at the tail end of the list." You think Mom is going to let you go to a party the seniors are throwing? Ha, don't hold your breath.

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.

don't hold your breath

used to indicate that something is very unlikely to happen.

with bated breath

Holding one’s breath back in expectation. To bate meant to restrain, but this verb is scarcely heard today except in this cliché, which itself has an archaic sound and often is used ironically. Shakespeare used it in The Merchant of Venice (1.3): “Shall I bend low, and in a bondsman’s key, With bated breath, and whispering humbleness.” A more recent colloquial locution is don’t hold your breath, meaning “don’t wait in vain.”