The Sun has a piece today on slang expressions used by different groups. Cheese and road dog are hip-hop terms for money and good friend, respectively. A haircut is a short sentence in prison slang and a gonkbag is a sleeping bag in army slang. The army is a good source of coined words and slang terms. I can barely understand any of the contributions on this army forum, where soldiers are discussing gonkbags (also spelt gonk bags). They're also called doss bags, jungle bags and desert bags, judging by the posts.
An onion is a sergeant in police slang. That's a modern example of rhyming slang (from onion bhaji, to rhyme with sargie). There's also a list of playground slang; hench means big and strong and a durbrain is a stupid person.
For more slang expressions see the article.