A sometimes derogatory slang term for someone either born or raised in Minnesota, Eastern South and North Dakota and Western Wisconsin that is sometimes also used as a badge of honor by said people. Derived from an alternate version of the children's game "duck, duck goose" played almost exclusively in the afore mentioned region of the United States. Supposedly, saying "grey duck" instead of "goose" allows for psych-out plays where you could say "greeen duck" or "grrrravel duck" or some such nonsense.
Alternate spelling: Gray Duck
Girl one "Where you from?"
Girl two "Sioux Falls, South Dakota"
Girl one "...oh."
Girl two "Yep, I'm a Grey Duck!"