Green’s Dictionary of Slang

1. a self-description, e.g. joe soap here ...

1980

Barltrop & Wolveridge Muvver Tongue 75: You lug me round the houses, get me to look at half the shops in Stepney – and when you feel tired, who gets the blame? Joe Soap!

2. a fool, a gullible individual.

1943		R. Llewellyn None But the Lonely Heart 35: Here I am, standing here like Joe Soap.
1947		J. Maclaren-Ross Of Love And Hunger 103: ‘My name ain’t Bleeding Joe Soap,’ I told her, ‘and don’t you forget it neither.’.
1979		J. Morrow Confessions of Proinsias O’Toole 131: Catch yerself on! – D’you think it’s oul’ Joe Soap yer talkin’ to?
1980		(con. 1930s) Barltrop & Wolveridge Muvver Tongue 31: ‘Joe Soap,’ for ‘dope’.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.

3. any man.

1980		J. McClure Spike Island (1981) 219: I’d rather do that than go and pinch Joe Soap over there for pinching a packet of fags.
2000		Indep. 27 Jan. 11: Edward’s qualifications wouldn’t get Joe Soap’s son into the local polytechnic.