Guns N’ Roses bassist Duff McKagan is convinced “The Simpsons” named Springfeld’s fave brew after him, but his theory is flat, ’cause Duff Beer was coined for a whole different reason ….

Here’s the deal … the rocker recently told Stereogum he is, indeed, Duff Beer’s namesake … going so far as to say the show reached out back in the day to clear it, and he’s bent out of shape he got cut out of merch profits. DMK repeated this belief in his autobiography, too.

But, ‘Simpsons’ OG producer-writer Jay Kogen tells TMZ Duff’s dead wrong for 2 reasons … one of which he’s gonna find very upsetting.

First, Jay says there actually wasn’t any deep reasoning behind choosing the name Duff.

But, secondly — and perhaps more insultingly — Jay says ‘Simpsons’ writers and producers back in the day didn’t know anyone in GN’R beyond Axl Rose! Sooo … sorry Duff?

He says, “We named it Duff because it’s a synonym for butt, tushy, booty, and so on.”

As for the rocker’s continued claim to the name, Jay adds … “It’s very weird this Duff McKagan guy wants to claim credit for Duff Beer … He had zero to do with it.”

Jay points out, there would have been nothing funny about naming the beer after a random rocker — “Duff is a beer for people who sat on their fat ass all day.”

Bottom line, JK says the Roses rocker has no more claim to Duff than Sir Mix-a-Lot does.