Similar to “extreme/extremist,” this term is often used pejoratively to frame an idea, person or institution as dangerous or ridiculous (and therefore, not worth paying attention to at all).  Often, the word is used in reference to political opponents or unpopular ideas as a way to shut down the conversation. For instance, someone who raises questions about climate change, evolution or vaccines has often been called a "fanatic" - dismissing the idea that there are any legitimate questions or concerns at hand that deserve to be explored.     

This is not to deny that legitimately extreme or fanatic ideas and people do, in fact, exist - only to highlight how the word has been weaponized far beyond the dictionary definition of a “person filled with excessive or single-minded zeal for a particular cause.” Indeed, the serious conversations that need to be had about truly dangerous extremism of any kind become greatly complicated in an atmosphere where otherwise reasonable disagreements are accused of being "extreme" or "fanatic" out of spite or anger.