In boxing

Boxing

Boxing, also called pugilism, is a combat sport in which two people fight each other using their fists. Boxing is supervised by a referee over a series of between one to three minute intervals called rounds...

, the "one-two combo" is a nickname of a jab

Jab

A jab is a type of punch used in the martial arts.Several variations of the jab exist, but every jab shares these characteristics: while in a fighting stance, the lead fist is thrown straight ahead and the arm is fully extended...

 and cross

Cross (boxing)

In boxing, a cross is a power-punch like the uppercut and hook. Compubox, a computerized punch scoring system, counts the cross as a power-punch....

 combo. It is so named because, in boxing parlance, the jab is often referred to as a "1" and the cross as a "2".

Boxers usually learn this combination before any other.

The jab is designed to get the fighter in range while effectively blinding the opponent and hiding the cross, which is a longer punch that will be slipped

Slipping

Slipping is a technique used in boxing that is similar to bobbing. It is considered one of the four basic defensive strategies, along with blocking, holding, and clinching. It is performed by moving the head to either side so that the opponent's punches "slip" by the boxer. This technique is...

if thrown on its own. Generally, the jab is thrown with 60% power while stepping forward into range, and the cross follows at full power as the lead hand is retracted back to the guard.

Many boxers use this combination of punches to weaken their opponents during the beginning of a fight before any other combinations.