For a word as mundane as mundane, mundane sure has nice origins! This word, now mostly defined as "boring" or "tedious", used to mean "of this world" in early modern English. The transition occurred because nothing going on in our plane of existence is nearly as interesting as the happenings of heaven or hell. Moving into other languages, mundane can be traced back to French mondain, "worldly", which further stems from the Latin word mundus, "universe or world", a term that is also present in the Spanish word for "earth", mundo, and the modern word map. This is where the etymology gets really fascinating: there are two proposed origins of mundus, each as unlikely as the next. The first theory is that is traces back to Etruscan muth, or "pit". The second notion is that it comes from the Proto-Indo-European word meuh, meaning "to wet" and is related to another PIE word meaning "elegant". I couldn't find out why these theories are so; the philological research on this was difficult to find. However, I'm inclined to think the origin is the latter, as many PIE words are etymons of Latin.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.