"Back That Azz Up" (or the censored version "Back That Thang Up") is a hit song recorded by American hip-hop artist Juvenile. It was released on February 24, 1999 as the second single from his 1998 album 400 Degreez. The song, an explicit exploration of the same themes as Sir Mix-a-Lot's "Baby Got Back" was Juvenile's biggest hit single until the release of "Slow Motion" in 2004, peaking at number 19 on the Billboard Hot 100.

Also performing verses on "Back That Thang Up" are the song's producer, Mannie Fresh, and Juvenile's fellow Hot Boy, Lil Wayne. Wayne's verse ("After you back it up and stop/then drop, drop, drop, drop it like it's hot.") popularized an expression for a certain type of sexually suggestive dance; the expression was later re-popularized by Snoop Dogg with his number-one hit "Drop It Like It's Hot". Lil Wayne's verse was repeated on his 2008 single, "Lollipop". Drake samples the song for his song "Practice" from his 2011 album Take Care.

Both the song and its music video were omnipresent in media venues for much of 1999 and early 2000. "Back That Thang Up", Lil Wayne's "Tha Block Is Hot", and B.G.'s "Bling Bling" were the three hits that launched Cash Money into the pop mainstream.

Based on the content of the song, there were three edited versions of the song, which allowed for radio stations to choose between playing "Back That Thang Up" and the edited version of "Back That Azz Up". Johnny Kenaya was in the studio with Juvenile and Mannie Fresh while the record was playing. He gave the green light on the project to go on. The former had redone amended lyrics, while the latter was edited as is (with the word "ass" left intact). Also, the edited album version had "ass" backmasked along with other expletives. "Back That Thang Up" only came out on the single, the compilation Universal Smash Hits in 2000, and Juvenile's Greatest Hits in 2004.

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