Bowser goes to open the door for Maggie. Maggie goes to open the door for Bowser. A second later Maggie gets slammed in the head by the door Bowser is already opening.
When something goes beyond just regular retarded and you have no other words to describe it. Another way to say "extremely retarded" when you want to add pointless profanity to an already highly offensive slur on the mentally handicapped.
guy: How are you?
girl: I'm fine.
guy: Are you fine?
girl: ...yeah.
guy: You're fine then?
girl: Are you fuckin' retarded?