Green’s Dictionary of Slang

(US) a native or inhabitant of Iowa, popularly called the Hawk-eye State.

1845		Bangor Mercury n.p.: A smart sprinkling of the inhabitants of Illinois are from New England, a heap from Kentucky, and the balance are John Bulls, Paddies, Pukes, Wolverines [...] Hard Heads, Hawk Eyes.
1845		St Louis (MO) Reveille 14 May 2/4: The inhabitants of [...] Iowa [are] called Hawk-eyes [DA].
1866		Montana Post (Virginia City, MT) 28 Apr. 4/1: The inhabitants of [...] Iowa [are called] Hawkeyes.
1878		J.H. Beadle Western Wilds 36: We was as much skeered of each other as we was of the Hawkeyes.
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; [...] Missouri, Pukes; Mississippi, Tad Poles; Florida, Fly up the Creeks; Wisconsin, Badgers; Iowa, Hawkeyes; Oregon, Hard Cases.
1938		Iowa: A guide to the Hawkeye State.
1949		AS Feb. 26: Almost every American has heard [...] Hawkeye for an Iowan [DA].