"God Rest You Merry, Gentlemen" is an English traditional Christmas carol, dated to the 16th century or earlier.
Roger decides to play with the adjustable light on his sweater and sings a hip-hop version of "God Rest You Merry, Gentlemen" in "The Most Adequate Christmas Ever" to pretend he's at a dance club while Stan insists on searching for the perfect Christmas tree.
G-G-God rest ye merry gentleman
I'm going to blow your mind
eh eh eh eh oh oh oh oh eeee eeee eeee eeee ahh ahh ahh ahh
Ping ping bong bong ping ping bong bong