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1.
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!

Dude2:Fosarelleus!?
by SantaMayCry July 02, 2005