Disco Girl

"Disco Girl" by BABBA is a knock-off version of the song "Dancing Queen" by ABBA, for the show Gravity Falls. It's played in three episodes of season one: "Dipper Vs. Manliness", "Land Before Swine", and "The Last Mabelcorn".

A full version of the song is available on the Gravity Falls official soundtrack, YouTube, and Alex Hirsch's Twitter.

(Sources: "Gravity Falls", and the "Gravity Falls Wiki")

"Have you heard Disco Girl?"
"Yeah, it's my favorite song!"

Mason knows all the lyrics to Disco Girl.