"Below the belt" is a type of idiom that is used to describe an action that is considered particularly hurtful and inappropriate. While several origins for the saying are advanced in different circles, the most commonly accepted relates the expression to a rule that is generally found in boxing. That rule, which prohibits throwing punches below the waist or belt line, is designed to place limits on behavior and keep the sparring on an even keel. As it relates to interaction in general, choosing to hit below the belt means to take advantage of a weakness of the opponent in order to successfully win an argument, a business deal, or achieve an end using methods considered outside the scope of fair play.
One of the easiest ways to understand what is meant by an idiomatic expression like "hitting below the belt" is to consider a situation in which a friend shares a deep dark secret with another friend. At a later date, the two parties become estranged. As a way of causing that former friend distress, the recipient of the information may choose to share that secret with people that both parties know, causing irreparable damage and embarrassment for the former friend. This type of activity would be considered unnecessarily hurtful or below the belt, and may even lead to some estrangement between the revelator of this secret and those that he wanted to isolate from the former friend.
Hitting below the belt can also sometimes be used in business settings. Here, the strategy usually involves methods that are considered unethical to gain advantage over the competition. For example, if the founder and president of a competing company is infirm and unable to function in his or her capacity, a competitor may take advantage of this situation in order to gain control of the business and eliminate the competition. In like manner, a business partner may, over a period of time, manipulate others in the partnership by forcing them to sell their interests or face public exposure of matters they would prefer to keep private. In any incarnation, hitting below the belt means using information that is not meant for public consumption or taking advantage of temporary weaknesses to crush an opponent.
It is important to note that there is a difference between simply making the most of situational opportunities and hitting below the belt. With the former, the strategies involves make ethical use of information that is in the public domain and readily available to others. The latter focuses more on the use of information that is not widely known and is likely to cause distress to the target if it is revealed with a certain type of spin or twist to the revelation. Hitting below the belt is not considered fair in any type of fight and those who choose this approach tend to enjoy short-term victories while limiting opportunities for the future, simply because observers will note the betrayal and use of proprietary data, and often distance themselves from the perpetrator.