because they fly at night [Italian: pipistrello, Slovenian: netopir, Polish: nietoperz, Greek: nykterides, Farsi: shab parreh]
So bat literally means flapper. You’re welcome.
This, my friends, this is true etymology. Explaining why something is named the way it is, finding patterns and principles of meaning, not just tracing a word’s form back through time (which, admittedly, is oftentimes a prerequisite for exploring the former).
This exact conversation is how I became friends with @pipcomix