First used on the Seinfeld episode "The Jacket" in 1991.
KRAMER: Hey. Hey, would you do me a solid?
JERRY: Well, what kind of solid?
KRAMER: I need you to sit in the car for two minutes while it's double-parked. I gotta pick up some birds.
Also used in the movie "Half Nelson" (Ryan Gosling)
"Juno" has caused a new wave of youngins to say this who have never used the term before seeing the movie, which has caused people who have used it prior to stop using it immediately. Take the term you fake ass scenesters.