"Joe Soap" is British rhyming slang[1] denoting a foolish stooge or scapegoat, Joe being an ordinary person, with Soap as a rhyme for dope.
The phrase appeared in a 1943 book of military slang by John Hunt and Alan Pringle: "Joe Soap, the 'dumb' or not so intelligent members of the forces.
The men who are 'over-willing' and therefore the usual 'stooges'."
The World War I song "Joe Soap's Army", sung to the tune of "Onward, Christian Soldiers", has the lyrics "Forward, Joe Soap's army, marching without fear, with our brave commander, safely in the rear."