Führer is a proper noun meaning "leader" or "guide" in the German language on Earth. As a title, Führer is most often associated with Adolf Hitler and the Nazi party, which had come to power during the 20th century, and who were ultimately defeated during the Second World War.
The word Führer was first used in connection with Hitler and the Nazi party after the Nazis seized control of the German government, especially after the death of German President Paul von Hindenburg. After the defeat of Nazi Germany in World War II, the word became unpopular due to its connection to Nazis.
Centuries later, when Federation historian John Gill attempted to institute a more benign Nazi government on Ekos, he named himself the Führer of the Ekosian state. His subordinate Melakon became Deputy-Führer, becoming Führer in all but name when he drugs Gill and seizes control of the Ekosian state. (TOS episode: "Patterns of Force")