A new surge of energy after a period of mental or physical exhaustion: “At the midway mark, the marathoner got her second wind and left the other runners far behind.” The expression refers to the fact that a person's metabolism changes to a more efficient mode during prolonged exercise.
Restored energy or strength, enabling one to continue an activity or task. For example, I wasn't sure how far they'd get in a week, but now they seem to have gotten their second wind and are making good progress painting the mural. This expression, dating from the late 1800s, was at first (and still is) used for returned ease in breathing after becoming out of breath during physical exertion such as running. It soon began to be applied to nonphysical efforts as well.