When you hate someone's girlfriend or boyfriend with a burning passion and your hate is mistaken for jealousy. The person with the annoying partner is usually blinded with emotion and refuses to believe that their partner sucks.
Guy: Dude your boyfriend sucks like hell. You can't stay with that douchebag.
Girl: Do I see some jealousy?
Guy: No, your boyfriend is just a big jerkwad. I can't believe you like him.
Girl: I think you're just jealous.
Guy: I heard you're jealous of Michael's girlfriend.
Girl: No, she's just a bitch.
Guy: Oh, denial jealousy?
Girl: Yeah. He refuses to believe she's a bad girlfriend.