Green’s Dictionary of Slang

(US) a resident of the state of Delaware, or anywhere flat.

1845		Cincinnati Misc. I 240: The inhabitants of [...] Delaware [are called] Muskrats [DA].
1866		Montana Post (Virginia City, MT) 28 Apr. 4/1: The inhabitants of [...] Delaware [are called] Muskrats.
1873		Jasper Wkly Courier (IN) 28 Nov. 8/1: Delaware Musk rats; Florida, Fly-up-the-Creeks; Goergia, Buzzards; [...] Maine, Foxes.
1885		North Amer. Rev. Nov. 433: Among the rank and file, both armies, it was very general to speak of the different States they came from by their slang names. Those from [...] Delaware, Muskrats.
1890		Century Mag. (N.Y.) July 369/2: Her grandmother [...] [had] a profound contempt for the ‘muskrats’ as the Flats people [near Detroit] are generally called [DA].
1909		J. Ware Passing Eng. of the Victorian Era.
1920		H. Wiley Wildcat 19: You triflin’ fiddle-footed mushrat!
1949		AS XXIV:1 26: To this list may be added [...] Fox for a Maine man, Muskrat for a Delawarean.