not hold one's breath

Example: “He says he'll repaint the fence this weekend, but I'm not holding my breath.”

Origin: The sarcasm rests on a hard physiological limit. A person can voluntarily hold their breath for a matter of minutes at most before the body overrides the decision entirely — the urge to breathe is driven by rising carbon dioxide and cannot be suppressed by willpower past a certain point. Telling someone not to hold their breath is therefore not advice against waiting in general but a precise claim about scale: whatever they are waiting for will not arrive within the only interval a human being can physically sustain, which makes it a considerably sharper way of saying something is unlikely than any direct statement would be. The phrase is American and twentieth-century, and it appears almost exclusively in the negative or as an instruction.