When having sex with a girl from behind, you gag yourself, making yourself vomit. Use the vomit to lube up your hand, and insert it into the womans ass, fisting her ass while you are doing her doggy style.
Last night, I gave Heather the DirtyDerek. She loved every minute of it.
When one masterbates with a dildo(originally with wooden dildo, but this is not necessary), up his/her (usually male and must have moustache)anus, and then pulls it out and whipes it on his moustache. This is known as a dirty moffatt.