Wash in this sense is "swill"

Interesting, 'cos I've never heard leftovers for pigs called anything but swill, occasionally "pig-swill".

"Slops" would be the leftovers on plates, normally, which magically transmute into swill once in the swill-bin.

I wouldn't associate "hogwash" with the sea, as does Rhuby, but then I wouldn't associate it with pig-swill either. Something that a hog "washes" itself in, perhaps (i.e. mud & crap) ? There's certainly an association with the supposed filthiness of pigs.

And hogwash definitely means rubbish/balderdash/tripe/bollocks on both sides of the Pond.