The Sythegore Arm was the title of the Orcish Horde's division of raiders. Blackhand was a member of the Sythegore Arm before rising to power. Blackhand was well honored by most orcs within the Horde. This is the only mention of the term. It may or may not apply to all orcish raiders.
Orgrim Doomhammer disbanded all raiders within the Horde after he took power because he feared that they would betray him in an attempt to put Rend or Maim on the throne (which was untrue; it was an attempt by Gul'dan to gain the Warchief's trust after his near-brush with death). By the beginning of the Second War, wolf-riders were no longer common within the ranks of the Horde's armies. Therefore, whatever the Sythegore Arm was, it likely stopped existing.