Sounds filthy (or at least naughty), doesn't it? It's not. It's a barrel and a bird.
The epic medieval tome "Gargantua and Pantagruel" by Rabelais is the source.
This could be the perfect lead in to an episode on 'Obscene and Obscure'. Not only is Gargantua and Pantagruel an obscure work of literature, it is full of hilariously dirty stories, parables and euphemisms. (Rabelais loved fart jokes!)
I'd like to hear suggestions from other QI fans about little known dirty stories, euphemisms, literature or history. Some possible jumping off points might be; Arabian Nights, Catharine the Great or Benjamin Franklin's "snuggle parties".