The term used by the actor who played Loki in both Thor and the Avengers, Tom Hiddleston. It is used to embody the trickster essence of his character Loki: the God of Mischief. Usually said aloud after one plays a silly or light-hearted joke on someone else. It originated from an MTV After Hours video where Tom Hiddleston and Josh Horowitz act out a little skit in which Tom plays a few dorky tricks on Josh (usually can be viewed on Vimeo).
Josh: -paying for his lunch- What, probably about ten bucks, right?
Cashier: Actually, it's been payed for already
Tom: -pops out from around the corner- EHEHEHEHE! You're such an idiot, I bought your lunch for you! ehehehe you've been loki'd again! LOKI'D!
Josh: ...okay, got me again...