Originally used for the "Vagina in a bucket" joke. Has morphed into everyday use for any strange joke.
person 1: ...then I found out it was a vagina in a bucket!!
person2: ROTFLAVIAB???!
person 1: what's the difference between a truck full of babies and a truck full of sand?
person 2: what?
person 1: you can't pick up sand with a pitchfork!
person 2: ROTFLAVIAB!!!!!!!!!!!!