a) Someone who ditches his mates early on a night is doing a back door boogie
b) A method of sneaking in clubs, thus avoiding paying the extortionate enterance fees, by kicking in the fire door and running straight into the middle of the dance floor so you cannot be spotted by the meathead bouncers.
"Dude i cannot be arsed paying £10 to get in here tonight"
"Fire door round the back, lets do a back door boogie"
When having sex with a young lady one withdraws his love staff at the point of ejaculation and sprays his love muck on the lady's chin. The jizz should start to hang down off the chin, creating a rather fetching white beard look, similar to Merlin's.
"Hey guess what Rich, I did a Merlin's beard on your mum last night!"