Green’s Dictionary of Slang

(US) a rebel, one who refuses to follow the party line.

1899–1900		C.L. Cullen Tales of the Ex-Tanks 70: Our new corrective electrical treatment for buckers and interrupters.
1907		C. M’Govern By Bolo and Krag 24: You know yourself what a bum soldier you gets out of a sorehead – a bucker.