Green’s Dictionary of Slang

sucker n.2

[? the state fish (Catostomus commersoni), the sucker; the sucking of much-needed water from the natural artesian wells; the gullibility of the early settlers in the hands of unscrupulous land speculators; for detailed discussion see R. H. Thornton, An Amer. Glossary, I pp.32–3 (1912)]

[mid-19C+] (US) an inhabitant of Illinois.