This term relates to what wearing of a thong does to one's booty. The simple act of wearing a thong results in butt flossage. This is very similar to the act of flossing one's teeth.
Fossage (FEE-kul sah-sij): AKA Fecal Sausage: The technique of making a sausage link like product using a filler and casing; in this case the filler is poop straight from the butthole into a condom as acting as the casing. In most situations the filler will unroll the casing as it’s pushed out. When complete the end can be tied off creating the fossage link, which can then be twisted into multiple smaller links for easier handling.
Billy needed to poop but was stranded in a nice place where he didn’t want to make a mess, with only a condom in his pocket he decided to make a fossage link and throw it inthe trash.