A phrase to describe someone who smells particularly bad, specifically of piss and dried ejaculate.
From the aprons Africans tie around the waists of their donkeys to stop them being able to penetrate and impregnate a female of their species. As you can imagine, a donkey's apron gets covered in all manner of gubbins.
Oh dear, dear fellow, you really must start to clean yourself after masturbating. You smell akin to a donkey's apron.
Stephen, stop fucking yawning and sighing all day long you fucking prick. Get some exercise, lose some weight and cheer the fuck up you fat tosser. By the way, you smell like a donkey's apron.