A song that you don't want other people to know you listen to, because it's extremely embarrassing.
"You're listening to Gimme More by Britney Spears? Dude, that's such an embarrassong. Better make sure it doesn't get scrobbled on your last.fm."
Any song by Britney Spears will fit the bill, as well as anything by boy bands or 1980s one-hit-wonders.