"Fat Lip" was written by Sum 41 members
Deryck Whibley,
Steve Jocz and
Dave Brownsound, and in-house producer
Greig Nori, with production by
Jerry Finn. The song gets its title from the
slang term for a swollen lip as a result of being punched in the face. "It was the last song I had written for
All Killer [No Filler]," Whibley told
Stereogum in 2021. "The whole album was pretty much done. It was never meant to be a single. It wasn't even supposed to be a song. The very, very first thing I wrote was the guitar riff. And I didn't necessarily write it for this idea that I had for this sort of punk rock-rap kind of thing. I knew I had this old school rap idea mixed with punk rock sort of stuff, but I wrote this riff just as a riff. And then I ended up writing a chorus, like, months later. And then I had this verse. And none of them were supposed to be together. They were just separate things that I was writing over time. And then one day it kind of clicked, and I thought, 'Well, these all kind of work. They're all around the same tempo, they're all the same key.' I changed a few things and made it work, now all of a sudden I was like, 'OK, I've got the rap part, I've got a riff, and I've got a chorus.' But I don’t have the rest of the song. And then it took a long time before pieces just kind of came together." The
uptempo song has been described as
pop punk,
skate punk,
rap rock and
easycore, with Whibley, Brownsound, and Jocz sharing vocal duties. "The verses are really about what we do: growing up in the suburbs, going to parties and hanging out with our friends, and causing trouble. A lot of people say they relate to it," said Whibley. Brian Hiatt of
MTV.com described the song as "pop-punk-meets-hip-hop",
Loudwire cited it as a containing elements of
hard rock and
PopMatters cited it as using elements of
heavy metal. The song has also been described to be "Mixing elements of skate punk,
nu-metal, and good old-fashioned pop." ==Music video==