bumper, n.⁶ — Green’s Dictionary of Slang

bumper n.⁶

[ety. unknown; ? they bump together in the pocket/one bumps it down on the table]

(US) five cents, a nickel.

1942		S. Kennedy Palmetto Country 135: And the cap’m didn’t fuss so very much when Daddy won a coupla bumpers [nickels] at it hisself [DARE].
1994		(con. 1930s–40s) C. Major Juba to Jive.