British slang dating from World War
II meaning to defecate, take a
dump or shit. The phrase came about due to the
heavy bombing of Dresden by the allies due to the concentration of military targets in and around the city. Dresden was also a centre of porcelain manufacture and since toilet bowls are made of porcelain, wartime humour made the connection, that and the societal requirement to use a polite euphemism, that if you went to take a crap you went to
bomb Dresden.