A "cheap shot" is an action that takes advantage of some vulnerability or exposure to cause excessive harm to another. This harm may take the form of physical pain, but in many cases the phrase refers instead to statements and remarks made to inflict emotional pain or to damage a reputation. An act is generally considered to be a "cheap shot" if it is unsportsmanlike or unfair.
In any confrontation, there are certain expectations or rules of engagement that opponents are expected to follow. A "cheap shot" will often violate these rules, allowing an attack to catch the defender off guard. Even when the letter of the rules is not violated, special circumstances can be exploited and trickery can be employed to violate the spirit of the rules. This is typically seen as cheating, and victories earned using this type of behavior may be considered illegitimate.
Sporting activities and other physical confrontations typically have clear rules outlining which actions are permitted. Even in less formal instances, such as a schoolyard fistfight, there are unwritten rules and points of etiquette dictating what is allowed. A maneuver which violates the letter or the spirit of these rules will typically be considered a "cheap shot".
Social situations may also be vulnerable to cheap tricks. Rules here are rarely formalized, but they are part of the culture and are typically well understood by all parties involved. At times, arguments can have a tendency to stray from the disputed topic, devolving into character assassination designed to discredit the opposition’s credibility or reputation. This is a very common "cheap shot."
When opponents are clearly mismatched, the weaker opponent might be given slightly more leeway. If an act levels the playing field instead of putting the defender at an overwhelming disadvantage, and if the severity of the act is not excessive, the maneuver might not be seen in a negative light, but rather as a clever tactic. This depends completely on the situation, the tricks employed and often the mindset of the viewer. Spectators might sometimes disagree on the validity of any such act.
The phrase “cheap shot” might seem slightly confusing, implying that the attack in question should be seen as “a good value”. In this idiom, cheap does not mean “inexpensive”. Cheap is used here to mean “of low quality” or “unworthy”, meaning that a cheap shot is beneath the dignity of all parties involved.