Instantly, quickly, or in an extremely short amount of time, as of a task or event. "Brace," taken from the old French for the arms' breadth from hand to hand, means twice; the phrase as a whole refers either to an old nautical term, meaning the time it takes the sail to shake twice as it takes up the wind, or else the short time it takes to shake a dice-box twice. I'll have that ready for you in a brace of shakes. We'll be there in a brace of shakes.