Reference to the John Candy movie "Summer Rental". When a person asks a dumb question or does something dumb, they are being a dumb swede.
Person 1: Oh no, I rounded off the head of this screw.
Rich: Righty tighty lefty loosey you dumb swede!!
From the movie "Summer Rental". Anyone who says or does something stupid can be called a dumb swede. It's a good insult for any occasion.
Frenchy: I rounded the head off this bolt.
Rich: Ahh you dumb swede!!