Green’s Dictionary of Slang

1. a petty crook, a tramp.

1919		N.Y. Trib. 25 Nov. 9/3: He also learned to gamble and became a ‘gandy dancer.’ A gandy dancer is a non-working inhabitant of railway construction camps who lives by an ability to ‘stack’ cards or ‘trim’ opponents with loaded dice.
1930		G. Milburn ‘The Dealer Gets It All’ in Hobo’s Hornbook 151: While some gandy-dancer warbled ‘The Wabash Cannon-Ball’.
1937		‘Boxcar Bertha’ Sister of the Road (1975) 9: Tales of the gandy dancers, and of the bindle stiffs [...] and the rides on the blinds.
1940		S. Longstreet Decade 349: Some clod-hopper or lumber-worker or gandy dancer [...] would eat crow.
1950		Goldin et al. DAUL 77/1: Gandy dancer. A cheap crook.

2. a jitterbug.

1942		Life 21 Sept. 44: Last week the nation’s needle nuts and gandy-dancers (jitterbugs) were cut to the quick [etc.] [HDAS].
1962		P. Crump Burn, Killer, Burn! 319: Three silk-shirted, reefer-high Mexican gandy dancers [...] strumming imaginary guitars.