Mainly used in combat sports such as 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...

 and kickboxing

Kickboxing

Kickboxing refers to a group of martial arts and stand-up combat sports based on kicking and punching, historically developed from karate, Muay Thai and western boxing....

, below the belt refers to all the area of the body that lies below the line of the opponent's navel, including the genital area, legs, and crotch. In these sports, as in many others, punch

Punch (strike)

A punch is a striking blow with the fist. It is the most commonly used attack in hand to hand combat. It is used in some martial arts and combat sports, most notably boxing where it is the only type of technique allowed...

es must not be struck below the opponent's navel, as it is deemed unfair and contrary to sportsmanship

Sportsmanship

Sportsmanship is an aspiration or ethos that a sport or activity will be enjoyed for its own sake, with proper consideration for fairness, ethics, respect, and a sense of fellowship with one's competitors...

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The expression is also used figuratively in to describe anything which is considered abusive, overly hurtful, or plainly unfair.