When a celebrity hooks up with a clearly "low-wattage" person (either another celeb, or a standard citizen), and said hookup causes the brighter celeb's star to dim. Opposite of upfuck.
As I finished my 4th cappuccino and started my second heart attack, a bit of downfucking walked by. Her name was Sheila and she was one of the cocktail waitresses from the Palm Beach Improv. See: Spears, Britney; Witherspoon, Reese; Hollywood, ALL OF.