This AA is called Gnoll Reaver Fortitude. It has a total of 5 ranks. It is given from the item Bloodmetal Inlaid Gnollish Totem.

This item can be obtained by doing the 15th Anniversary quest in Plane of War, and can be ranked up by leveling up Gnoll Spirit Shrouds in PoKnowledge (see Yaldu Bwor). It maxes out at Rank 5 and 48AC (On a Warrior, other classes receive less AC).