A song that everyone loves and sings, but no one actually knows the lyrics to.
They'll mumble along, or sing something that sort of sounds like the lyrics. Sometimes they'll actually believe what they invent.
But most of the time they won't admit they don't really know the lyrics.
Girl 1: In When I Grow Up by The Pussycat Dolls, are they saying "I wanna have boobies," or "I wanna have groupies" or what?
Girl 2: Sweetie, no one knows. That song is such a singalone song!