an acronym for Shanked With A Bullet
Due to the superior feeling caused by stabbing someone with a knife in the gaming world, swab or swabbed is used to ridicule an opponent after an especially superior kill. Since a shank is essentially a pointed object, to swab someone basically means to shank them at range.
ch00bs4uc3: Dangit I died!
xXxNubSlayerxXx: Oh baby, you've just been swabbed!