Meaning

noun `Johnny' was applied as a nickname for Confederate soldiers by the Federal soldiers in the American Civil War; `greyback' derived from their grey Confederate uniforms

noun a person who takes part in an armed rebellion against the constituted authority (especially in the hope of improving conditions)

noun someone who exhibits great independence in thought and action

originFrom Middle English rebel, rebell, from Old French rebelle, from Latin rebellis (“waging war again; insurgent”), from rebellō (“to wage war again, fight back”), from re- (“again, back”) + bellō (“to wage war”).

Etymology from Wiktionary, CC BY-SA 3.0