A term derived form Comedy Central airings of "The Big Lebowski." It was used to substitute "Fuck a stranger in the ass" in the infamous "you see what happens" scene. It means fuck a stranger in the ass. It is used in conjunction with "find a stranger in the alps."
Usually in a comedy situation with two people, one is the "straight man". He sets up the funny guy with a "straight line" -- a statement which allows the funny guy to make a joke. Therefore a straight line is really a "set up" for a joke.