A moron or simpleton. As an ex-employee of the St. Louis factory that was the subject of the above song put it: “â€˜Jellybean’ means you half-cracked. . . . If I call you a â€˜jellybean’ that mean you a simple-crack guy. . . . If a guy call me a â€˜jellybean’ direct to my face, he callin’ me somethin’ ignorant . . .” (Sylvester Grant, 1970 interview with author). As a general pejorative, the term dates to about 1915 (DAS); it appears in Faulkner’s The Sound and the Fury (1929).
From Barrelhouse Words: A Blues Dialect Dictionary by Stephen Calt.