Green’s Dictionary of Slang

1. filth; thus a piece of excrement.

1865

Leaves from Diary of Celebrated Burglar 74/1: Ye’d better mind yer shoon, for there’s ann awfu’ heep o’ dubs i’the ‘close’.

2. (Aus./N.Z., also dubs) a lavatory [+ abbr. of double-you see, i.e. W.C.].

1943		‘Mrs E.F. Boswokick’ Amateur 9: Privy’s a new name to us out here, bein’ as we was all brought up to call it the Dub.
1966		D. Niland Pairs and Loners 88: I was in the dub dying when a flattened cigarette packet came sliding under the door.
1979		E. Smith Saddle in Kitchen 16: The archaic pit-style dub stood some distance from the house on the bank of Ego Creek.
1980		Barltrop & Wolveridge Muvver Tongue 39: ‘Lav’ was a reasonably polite word [...] So was ‘w’ short for ‘w.c.’. The latter was also abbreviated to ‘dubs’ by children.