1. Two quick punches delivered to one's opponent in rapid succession, one coming from each hand. The boxer's put his opponent in a daze with a flurry of punches, then brought him down to the mat with the old one-two. The guy refused to leave us alone, so I gave him the old one-two and shut him up for good.

2. Two actions, events, strategies, people, etc., that work in combination to deliver a very effective or powerful result. The two lawyers have become the one-two punch of the firm, bringing a perfect combination of abilities to tackle any case. The storm is predicted to bring the old-one two—devastating winds and a deluge of rainfall.