A shit box is a person who is shit, kind of like the shit heads who go 0-10-7 and let their carry die in dota, and then when they win because they get carried, they ego you. Shitbox is usually a term used to insult shit players to leave them mentally scarred so they never play again, it tends to make them instant leave if you call them shitbox enough.
master_pain: u suck dick, ima fucking rape u asshole u dick sucker
trjial: you're a shitbox, you know that? you fucking let invoker down, u shitbox, u fucking suck, shitbox.
trjial: you're a shitbox
master_pain: -left the game-