Green’s Dictionary of Slang

(US) a nickel (five cents).

1916		P. Kyne Cappy Ricks 345: Let me play the game of business with you, son, down to my last buffalo nickel.
1929		S.J. Perelman Dawn Ginsbergh’s Revenge 187: I suppose you feel like Andrew Carnegie every time you yank a buffalo out of your pants and throw it to me like a fish to a seal.
1936		R.L. Bellem ‘Falling Star’ in Spicy Detective Sept. 🌐 I dived for the phone-booth. I dropped a buffalo; dialed the home number of Sid Grainger.
1944		J.M. Inks diary Eight Bailed Out (1954) 3 Dec. 119: I wished I had a buffalo nickel.
1954, 1965–70		in DARE.
2001		S. King Dreamcatcher 415: Dunno why you’d want to talk to that meringue-head, but it’s your buffalo nickel.