ˌone-ˈtwo noun [countable]
a movement in which a boxer hits his opponent with one hand and then quickly with the other:
Ali gives his opponent the old one-two, and it’s all over.

[TahlilGaran] Dictionary of Contemporary English ▲

one two
n.
1. A succession of two punches, the first a short left, followed by a hard right punch, usually in the jaw.
Ali gave Frazir the one-two.
2. Any quick or decisive action which takes the opposition by surprise, thereby ensuring victory.
He gave us the old one-two and won the game.

[TahlilGaran] English Idioms Dictionary ▲