An Americanism for what in proper
English is a "bum bag" — a pouch or other device worn around the waist by butt sluts to hold safer-sex supplies, sex toys and other paraphernalia for
kinky buttsecks. The wearer is typically clad in skintight stretch clothing to showcase their assets, leaving nowhere else to store their anal lube, condoms, butt plugs, douche or enema bulbs and other supplies needed to prepare their butts for
anal sex. While jogging (running around) or cycling (peddling their arses all over town) to find a mate, the savvy buttslut comes prepared by wearing this
pack (usually directly over or above the trampstamp) with everything needed to savour the fruits of Sodom.
So why, then, would a bum bag be a "
fanny pack"? Most likely, a few trollops are looking for a Devil's threesome which involves double penetration — one penis in the
bum, another penis in the
fanny at the same time. That would require extra rubbers, extra lube and presumably redundant birth control — all of which needs to be packed away in that little pouch.