Green’s Dictionary of Slang

(US) an inhabitant of Maine.

1845		St Louis (MO) Reveille 14 May 2/4: The inhabitants of Maine, are called Foxes [DA].
1866		Montana Post (Virginia City, MT) 28 Apr. 4/1: The inhabitants of [...] Maine [are called] Foxes.
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 Maine were called Foxes.
1909		J. Ware Passing Eng. of the Victorian Era.
1949		AS XXIV:1 29: Fox for a Maine man seems to embalm an early impression that the citizens of the state were smart fellows.