A SATA buddy is another character that a thief (from a game such as FFXI) stands as closely behind as possible when using the abilities "Sneak Attack" and "Trick Attack" (a.k.a. SATA). The character in front of the thief absorbs the hate generated by the SATA and, consequently, the impending damage generated from the mob (monster in the case of FFXI). This move is done when a secondary character grabs the attention of the mob and turns it so that the SATA buddy and the thief are directly behind the mob.
A paladin stands directly in front of a thief and a warrior provokes the monster. When the monster turns to attack the warrior and the thief uses SATA. When the monster turns back around, the attention and hate resulting are focused on the paladin, who is in this instance the SATA buddy.