When one, after the act of love-making, proceeds to shit in the sheets. They then wrap their sexual partner up into a bag using the soiled sheets. Next, the bag is thrown over the shoulder in a Santa-like fashion and shaken until utter orgasmic splendor is reached.
you: "Dude I totally went for the Santa Bag last night! She was so turned on she shit herself!"
friend: "DOUBLE SANTA BAG!!!"