Get a Second Wind

Describes a genuine, documented physiological event. "Wind" meant breath in English as early as the 14th century, and the specific phrase is recorded from the late 1800s, describing the real experience of a runner's laboured breathing suddenly easing during sustained exercise. That easing is a real physiological shift, the body switching from anaerobic to more efficient aerobic respiration, which happened to already have a name before science fully explained the mechanism behind it. A popular nautical theory, that it refers to a becalmed sailing ship suddenly catching a fresh breeze, sounds plausible but has no historical evidence behind it.