Someone who knows his girlfriend is cheating on him, or at least messing around with a bunch of other guys, but chooses to ignore it because he can't resist her.
Made popular by The Killers's song, "Mr. Brightside."
"Dude, Jeffery's girl is over theremaking out with some guy! Does he know about it?"
"Yeah, but he's a Mr. Brightside, so he just ignores it."