SwaG is swamp vag, as in swamp vagina, akin to swamp ass or swass. Otherwise known as a horrible vaginal odor. Unlike swag (swagger, free stuff, etc), SwaG is pronounced with a hard G (as in gentle).
After an afternoon of running errands and hitting the gym, my morbidly obesegirlfriend developed some serious swaG, but that didn't stop me from gobbling on her moist stink-muffin.