VIDEO: Watch Matt Damon's Surprise for Jimmy Kimmel

02/01/2008 at 01:45 PM EST

(Warning: video contains graphic bleeping)

Comedian Sarah Silverman had a special surprise for boyfriend Jimmy Kimmel on Thursday night's 5th anniversary of his ABC late-night show – and it involved getting naughty with Matt Damon.

In what has become an instant YouTube hit, Silverman details her faux-adulterous behavior with the reigning PEOPLE Sexiest Man Alive, singing, "I'm f---ing Matt Damon... I'm not imagining it's you."

Strapped with a guitar, the potty-mouthed Silverman confesses that she's rolling in the hay with Damon while Kimmel is "drinking Diet Snapple." Damon (a Kimmel pal and frequent joker on the late-night show) gets in on the fun, too, asking Kimmel, "How do you like them apples?"

Damon and Silverman, Kimmel's girlfriend of over five years, even show off their dance moves in a hip-hop interlude.
–Christina Tapper