Scotty doesn't know is a song by Lustra. It features in Eurotrip, which is a fairly good film, but should only be watched if you don't feel like thinking much.

Scotty is the main character of the movie, and in the opening scene his girlfriend breaks up with him. He then goes to a party, determined that Fiona will not ruin his graduation night. At the party there is a band. The singer declares that his next song is for "the craziest little sex puppet he knows". Fiona shoves through the crowd and they stick their tongues down each others throats.

Scotty doesn't know,
That Fiona and me,
Do it in my van every Sunday.

She tells him she's in church,
But she doesn't go,
Still she's on her knees, and

Scotty doesn't know
Scotty doesn't know
So don't tell Scotty!

The basics of the song are, of course, about the affair that the singer and Fiona have. The lyrics basically praise how sneaky they are and how tease Scotty for his trusting nature:

Fiona says she's out shopping,
But shes under me and I'm not stopping.

I can't believe he's so trusting,
While I'm right behind you thrusting.

Fionna's got him on the phone,
and she's trying not to moan.
It's a three way call,
and he knows nothing.

The, parking lot, why not?
It's so cool when you're on top.
His front lawn, in the snow.

I did her on his birthday.

Scotty's gotta go!

There is also an extreme use of the phrase "Scotty doesn't know!". The song was written for the movie and is performed by Lustra in the film, but Matt Damon got to act the part of the singer. The song is the theme song of the film and is repeated quite often throughout it, including at a nightclub and as Cooper's ringing tone. Apparently the song has gotten so popular it is available as a ringtone.

The song ends with various people shouting "Scotty doesn't know" in a way that makes you think of marching protesters and fists punching the air. It then ends at 2.57 minutes with laughter, as if to say Haha, I'm sleeping with your girlfriend, Scotty.

Even though, in the movie, you hate both the singer and Fiona by the end of either the song or the movie you're singing along and are just as determined as everyone else to yell out that Scotty doesn't know. It's extremely catchy, and just a fun song in general. Go listen.