An unsuccessful player. In the dice game of chuck-a-luck, backroom players tossed the dice not with their hand but out of a small metal handheld cage called the “horn” (more upscale games used leather horns). Hence, the “tin horn” noun that became the “tinhorn” adjective when applied to nickel-dime gamblers. “Tinhorn” sounds as though it might also refer to a musical instrument, and composer Frank Loesser took advantage of that sound-alike association with “Fugue for Tinhorns” in his musical Guys & Dolls.
See also: gambler