The word originated from the word Fo' Realious (Foe-Real-ee-us). But when one 12 year old on a halo 2 clan forum misspelled it, the word became Forsarelleus (Foe-sa-rel-ee-us)
It is used to verify something that someone says. Its very similar to saying "seriously"
Dude1:Man, i teabbaged the shit out of him!