1 definition by Mr. Perversion
A Backdoor Milkshake is when one proceeds to insert any sort of liquid into the rectum of another person via any means, then the recipient of said liquid proceeds to fart out the liquid, leading to a squirting phenomenon.
Two lesbians enter a bathroom. One lesbian pulls out a metal syringe full of milk, the other bends over, and receives this gift of strong teeth and healthy bones right up her ass. She then squirts it out of her bum and the other lesbian catches the milk in her mouth. That's a Backdoor Milkshake.
by Mr. Perversion January 28, 2009