1. The act of masturbation which involves a glass shower door and several litres of blood and semen (ratio of blood to semen differs from person to person and frequency of said act). Typically performed when regular meat beating just doesn't cut it.
See also furious masturbation
2. An extremely angry nocturnal winged mammal.
Ex 1. "John? What are you doing in the bathroom? You've been in there for over half an hour. You're not having a furious bat are you?"
Ex 2. "Dude, I went into the cave, didn't find any water, only furious bats."