hold one's breath
Be excited, anxious, or nervous. For example, The election was so close that I held my breath until the final results were in, or I'm holding my breath until every-one's been heard from. This expression alludes to the interruption of normal breathing; the literal usage dating from the early 1700s.
not hold one's breath. An expression used to mean one is not awaiting something, as in I'm hoping to hear if I got the job, but I'm not holding my breath. It often is put as an imperative, don't hold your breath, meaning "don't expect it, it's not likely," as in They may get married this summer, but don't hold your breath. This expression in effect implies it is unwise to stop breathing until a particular event occurs, since it may never come to pass. [Slang; mid-1900s]