As I understood it back in the old times…. The Flat Earth Society was a groups of logical rationalist who got together and for shits and giggles (as well as practicing rational debate) would pick an indefensible argument (eg. the earth is flat) and attempt to use rational arguments to support that indefensible claim.

Basically a rowdy drunk philosophers get together.

Edit: there is no proof of this, just something I read online in the early days of the internet.