So where does this odd little word come from? It is thought to be derived from the middle english word for the bat: bakke, possibly as a result of confusion with the latin word for a night-flying insect: blatta. Bakke itself is likely to be a shortened version of the old Danish word for a bat: natbakka, or "night flapper". Sticking with our Scandinavian forebears, the old Norse had a wonderful name for them: leorblaka, which means "leather flapper". That name must come from someone who had seen a bat's wing close up. It's such an evocative name it's almost worth reviving it!
Other forebears of ours (an awful lot of old races contributed to Britain over the years) also had some interesting names for the bat. The old english word for bat is hreremus (pronounced rear-mouse), meaning "shaky mouse". Whoever came up with that name had certainly watched a foraging pipistrelle!
When the Romans came to Britain they brought the latin word for bat: vespertilio, derived from vesper, their word for "evening". From this comes the family name for all our British bats, apart from the two horseshoe bat species: vespertilionidae. It's also the original root for another bat word. In old Italian vespertilio became vipistrello and thence the modern Italian word for bat: Pipistrello, which of course led to Pipistrelle.
Other modern European languages have interesting names for bats. The Germans say fledermaus, literally "flying mouse". For an ecologist, trying to convince people to live in harmony with bats, it's a little disappointing that so many names for bats refer to mice, at risk of tainting bats with their image as vermin. I suppose it's inevitable.
The French use the word chauve-souris, which translates literally as "bald mouse" This seems odd, until we discover that it's derived from the ancient Greek calva sorix, meaning "owl mouse", which makes more sense.
For me the prize must go to the Spanish. Their name for the bat is derived from the latin mures caeculus or "blind mouse". That's bad on two fronts, as the bat is neither blind nor a mouse, but all is forgiven, because in modern Spanish the word becomes murcielago, the name given by Lamborghini to one of the fastest and most lavish supercars in the world. That's more like it!