`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
used by Northerners of Confederate soldiers; "the rebel yell"
take part in a rebellion; renounce a former allegiance
a person motivated by irrational enthusiasm (as for a cause); "A fanatic is one who can't change his mind and won't change the subject"--Winston Churchill
marked by excessive enthusiasm for and intense devotion to a cause or idea; "rabid isolationist"