South African slang for cheap, rotgut wine. From 'doos' which means cunt. Doos also refers to a box, so the term 'dooswyn' may be used in polite company to refer to boxed (as opposed to bottled) wine, but most will see the duality in meaning.
THEY ONLY SERVE DOOSWYN AT EXHIBITION OPENINGS
av moedlin 18. oktober 2011