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

originBorrowed from Russian РЭБ (REB), abbreviation of радиоэлектронная борьба (radioelektronnaja borʹba, “radio-electronic combat”).

Etymology from Wiktionary, CC BY-SA 3.0