For the past few weeks, an ethnomusicological mystery has gripped WXPN, where we make World Cafe: What exactly is butt rock?

It’s a question decades in the making, but interest in the subgenre has been growing. Earlier this year, media outlets like Rolling Stone and CBC heralded the renaissance of butt rock.

Here at WXPN, not everyone seems to agree on its exact definition. Anecdotal data collected around our Philadelphia-based station suggests that there are at least two schools of thought, split along generational lines. On one side, there’s World Cafe contributing host Stephen Kallao.

“As a kid who was born in the late 1970s, [butt rock] is a very specific time for music,” Kallao says. “It’s late ’70s, early ’80s … very machismo, hyper-sexualized cousin of glam rock.”

Kallao says the debaucherous excess of bands like Winger, Poison and Mötley Crüe are the epitome of butt rock.

To his point, a more pejorative use of the phrase goes back to at least the early ’90s. Soundgarden drummer Matt Cameron is frequently cited online for his use of “butt-rock” in a 1994 book on the grunge band.

“Butt-rock is just the stupidest, basic, three-chord rock that you can possibly play,” Cameron says in Soundgarden: A New Metal Crown.

Then, there’s the popular copypasta that attributes the term to a “nationwide advertising campaign on hard-rock radio stations in the 1990s.”

But for Millennials and Gen Z, butt rock is a 21^st century phenomenon.

“In my mind, it’s early 2000s,” says World Cafe producer Miguel Perez. “It’s post-grunge. It’s Nickelback, Creed and Puddle of Mudd.”

For these generations, “butt rock” is a term of endearment for the post-grunge bands that dominated radio airwaves during their childhood. Plus, Nickelback’s Chad Kroeger and Creed’s Scott Stapp have long served as a bedrock of the Millennial meme economy.

In some ways, these subgenres are simply successive waves in the terrifying ocean that is hard rock music. The sound may change from decade to decade, but the spirit of commercialism and machismo remains.

“In the ’80s, there’s all this debauchery and everyone’s living high off the land. It’s all good times and ‘sex, drugs and rock and roll,’ ” Kallao says. “In the 2000s, we’re in a war. There’s been a national tragedy. As a result, that shifted the tone. It got a lot darker.”

Listen to World Cafe‘s butt rock mix:

Angel Olsen: “Spring” (World Cafe Session)