When you go to town with your friends then go to get a piercing but actually go to an area that is known by the squad 'the den' and have sexual intercourse. This usually results in leaves in your trousers.
The 'Dens' refer specifically to an area in North West London. Namely Willesden, Harlesden, Neasden. I live in the Dens and I'm proud. Just about the most multicultural mix anywhere in London.
Check out Unique FM 101.2 on your dial serving the Dens and beyond.