Urban Dictionary: Best In Slot (BiS)
Best In Slot is used by gamers to define a piece of gear which is simply the best for their gaming character in a specific slot (eg. head slot, pants slot etc.) Also someone's "Best In Slot" is his/her best friend, a very special person that you usually meet online through an mmo game and end up being very close to each other and very important for each other just like a BiS item. Sometimes though your friendship can be "vendored" in case your partner finds an "upgrade".