Slang for "taking a dump". Coined by the nefarious Jeff, when his friend Sam mentioned on gchat that he was going to be afk while he started "unboxing the couch". However, mid unboxing, Sam needed to release his bowels.
Since pooping could be seen metaphorically as "unboxing the couch", the term then became synonymous with pooping.
Dude, that curry did a number on me. Excuse me while I go unbox the couch.