"Theorycraft" originated from the PC game Starcraft. The term was coined after countless new players would argue with each other about how certain strategies were superior to other strategies without actually putting any of these concepts into practice. This later evolved into a game in itself, albeit sarcastically. From Starcraft to Theorycraft.
Player_2: Pfft, theorycraft
"Yeah I'll believe that when it isn't Theorycraft."
Nerd 1: if ur a X n u Y every Z u fight u omgwtfpwn them every time, lolz!!!111eleven
Nerd 2: Well considering Z's don't exist yet, that's theorycraft
Nerd 1: I'm sick of theorycrafters, don't they have anything better to do?
Nerd 2: Not really