This term, frequently used by non-sailors to refer to sailors, is most likely a contraction of the term "swabbie" or "swabby", presumably derived from the notion of sailors "swabbing the deck". "Swabbie/y" is currently in use only among the elderly; "swab" is the more common term among young people.
Typically applied to sailors of the U.S. Navy.
The past tense would be "Swabbed".
Angela: "I'm so horny, I think I'm gonna go swab."
Marcus: "I was swabbing this girl the other day, and she totally had herpes!"
Jaime: "It smells like tuna in here!"
Katie: "Sorry...I just swabbed."
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.
xXxNubSlayerxXx: Oh baby, you've just been swabbed!