Green’s Dictionary of Slang

(US) an inhabitant of Illinois.

1835		Western Rev. June 342: There is neither wit nor meaning in the terms Hoosier, Sucker, Corncracker, and Buckeye, which have become so current [DA].
1838		T. Haliburton Clockmaker II 264: There’s the hoosiers of Indiana, the suckers of Illinoy, the pukes of Missuri.
1843		W. Oliver Eight Months in Illinois 94: They may be a little too ‘slick’ for the Illinois suckers.
1848		Bartlett Dict. Americanisms 343: sucker. A nickname applied throughout the West to a native of Illinois. The origin of this term is as follows: The Western prairies are, in many places, full of the holes made by the ‘crawfish,’ [...] which descends to the water beneath. In early times, when travellers wended their way over these immense plains, they very prudently provided themselves with a long hollow weed, and when thirsty, thrust it into these natural artesians, and thus easily supplied their longings. The crawfish-well generally contains pure water, and the manner in which the traveller drew forth the refreshing element gave him the name of ‘Sucker.’—Letter from Illinois, in Providence Journal.
1856		R.W. Emerson Eng. Traits 27: I found abundant points of resemblance between the Germans [...] and our ‘Hoosiers,’ ‘Suckers,’ and ‘Badgers,’ of the American woods.
1862		Congressional Globe 30 Jan. 566/2: I never before knew a ‘sucker’ who would not contend that we [i.e. Illinois inhabitants] could do anything and everything [...] better than other people.
1871		Semi-Wkly Louisianan 31 Aug. 1/3: The Nicknames of the States [...] Illinois, suckers; [...] Ohio, buckeyes.
1885		Whitman ‘Sl. in America’ in North Amer. Rev. Nov. n.p.: Those from Maine were call’d Foxes; New Hampshire, Granite Boys; Massachusetts, Bay Staters [...] Illinois, Suckers; Missouri, Pukes; Mississippi, Tad Poles.
1893		W.S. Walsh Literary Curiosities 1039: Though Illinois does not specially abound in ‘suckers,’ and ‘badgers’ are rather scarce in Wisconsin, the two commonwealths are still respectively known as ‘the Sucker State’ and ‘the Badger State’.
1901		W. Churchill Crisis 152: The Tall Sucker was on the steps to receive them.
1915		M.G. Hayden ‘Terms Of Disparagement’ in DN IV:iii 200: ‘The suckers of Illinoy, the pukes of Missouri, and the corn-crackers of Virginia.’ (Thorn).
1948		Daily Ardmoreite 11 July 21/5: Missourians have rejoiced in the name of ‘Pukes’ and Illinoisians as ‘Suckers’ [DA].
1989		H. Rawson Dict. of Invective (1991) 377: Sucker has enjoyed an inordinate number of other meanings over the years, among them: [...] (4) a lollipop or all-day sucker; and (5) a traditional nickname for an inhabitant of Illinois.