Noun. The kind of huge, vile, cancerous-looking greeny coughed up and spat out onto the ground by British Naval dockyard workers. Called an oyster because of the solid, greeny, snotty, rubbery centre surrounded by a lighter, watery fringe of phlegm, and the fact they just CLING and stick around for ages. British sailors have inventive imaginations, so you couldn't expect this to go un-named for long.
Officer at the gangway: Bo'sun's Mate, what's so funny?
Bo'sun's Mate: It's Able Seaman Smith, Sir, He slipped on a dockyard oyster when he was crappers last night and broke his leg! In front of the Captain's wife! Fucking brilliant!