dear dr. bill: there's an expert for everything! My source:
"The name Chicago is derived from the local Indian word Chicagoua for the native garlic plant (not onion) Allium tricoccum." Copyrighted paper by John F. Swenson (adapting prior paper), published in Early Chicago, Danckers & Meredith (2000), p. 377 ff. See penultimate entry at:
http://www.northbranchrestoration.org/art0199b.htm