Jimmy Kimmel -- BLASTS Jay Leno ... He's an Unfunny, Sellout

Jimmy Kimmel's hatred of Jay Leno continues to fester ... so much so, he RIPPED the late night talk show host a new one ... calling him a "sellout" who hasn't seen a laugh in years.

Jimmy unleashed a flurry of insults in a new interview with Rolling Stone ... telling the mag "Leno hasn't been a good stand-up in 20 years."

For the nail in the coffin, he adds ... "As a comedian, you can't NOT have disdain for what he's done. He totally sold out. He was a master chef who opened a Burger King." Snap.

FYI -- Kimmel's had beef with Leno for years ... starting way back in 1992 when Leno beat out Kimmel's comedic hero David Letterman for "The Tonight Show" gig.