The phrase “don’t hold your breath” is an English idiom, or saying, that means that something isn’t going to happen, won’t happen for a long time or won’t happen as expected. This is based on the idea that something will take so long to happen that holding one’s breath in anticipation would be impossible. It is slightly sarcastic and implies that either the initial statement before the saying is naive, or that the person or mechanism required to bring about the desired situation is incapable of doing so. To be told that someone is not holding his or her breath is essentially an insult.
One situation in which this idiom would be appropriate is when there are high expectations. If one speaker says he or she is expecting something to happen to a certain extent, such as a construction company finishing a building in record time, another speaker could say, “Don’t hold your breath.” This would imply there are circumstances that will prevent the company from completing the work in a shorter-than-normal time frame. It also implies that the circumstances are not pleasant, such as it being physically impossible to complete a building in the amount of time expected.
Another meaning is that something will take a much longer time than it normally would. If that construction company is known for delaying projects or taking an unusually long time, a speaker could use the phrase to imply that anyone expecting a job to be done within a normal time frame is kidding himself or herself. Finally, the saying “don’t hold your breath” could mean that something is not going to happen at all. To use the example of the construction company, if the first speaker says that he or she expects a project to be completed but the construction company has a record of disappearing with project money before starting work, the second speaker could use the phrase to imply that the first speaker shouldn’t expect to see anything done. In all cases, an explanation usually follows the phrase.
A literal meaning for “don’t hold your breath” does exist, and the context of the situation enables speakers to distinguish which meaning is appropriate. For example, a weightlifting instructor who is teaching proper form might tell a client to not hold his or her breath while lifting the weight. In that case, the sarcasm of the idiom wouldn’t make sense, but the literal meaning of continuing to breathe instead of holding in a breath would make sense.