Refers to the Special Forces tab, which is awarded to soldiers who have successfully completed the U.S. Army's Special Forces Qualification Course, or "Q-Course." The term "long tabber" refers to a soldier who wears the Special Forces tab as well as the Green Beret - Generally bad-ass, combat-hardened, mother fuckers.
Hey Bill, don't fuck with that guy... he's a long tabber!
Some private tried to hit one of the long tabbers with a wooden sword - we had to get that dumbass off of Fort Bragg and quick, before he got killed!
Ladies love the long tab.