short for Protoss Arbiter from the PC game, StarCraft. The flying vessels provide invisibility to surrounding friendly units. They can stasis units to protect them from any and all damage, and can use recall to teleoprt units anywhere on the map.
Holy crap dude, that Arb just recalled 8 Reavers into my minerals.
An epic typo by Mabinogi NA player "archranger" of the Alexina server. This word was meant to be "Narb" but he failed so epicly it turned out as "Arb" and the term slowly developed into the frequently used word we know today, ARB!