The term "underground fighter" usually refers to someone who participates in illegal or unofficial fighting matches, without the use of sanctioned fighting organizations. Some common synonyms for this term might include "street fighter," "rough fighter," "bar brawler," "unlicensed boxer," or "back alley fighter." These individuals are often seen as tough, gritty characters who have a deep need to prove themselves or defend their honor through fighting. However, because these types of fights are usually much more dangerous and unregulated than sanctioned events, underground fighters can face serious health risks or legal consequences for their actions.