Also called fighting dirty is an action or actions that are considered 'dirty,' deliberate, unnecessary or excessive because they are perceived as unethical, unfair, dishonest or a 'last ditch desperation move' ways for one to gain favor or the upper hand in a scenario for themselves or their cause as oppose to "taking the high road and fighting fair". These are aimed at something vulnerable, sensitive or unprepared for the damage.
These actions are usually considered to be easy, crafty ways to get a desired result and may involve breaking written and social rules or involving things that aren't even relevant to the issue at hand if convenient to quickly make things difficult for the opponent and 'take heat off themselves'. They may also be used when the user simply has no legitimate or fair moves, points or reasons to get their way and desired victory or glory or might not even deserve it, but to someone willing to use a cheap shot "a win is a win, even if everyone else thinks I'm a scumbag. They're just jealous!"
In Avatar the Last Airbender Azula zaps Aang in the back while in trance, nearly killing him and unknowingly nearly blocking him from ever being able to return to the Avatar State again. She later attempts to zap Katara while fighting Zuko, though Katara was not involved or on guard, but Zuko took the attack nearly to his heart which might have been fatal. Thus Azula assured herself victory over at least one of her enemies.