There have been whispers of an organization known as the Guild of Thieves, however, any details of their existence and abilities known by them are merely rumor. If they do actually exist, this might have something to do with them. Or maybe not. There is no guarantee of the accuracy or validity of anything below.
Sap is believed to be an ability used by members of the Guild of Thieves.
The thief attempts to thwack a target on the back of the neck in an attempt to knock them unconscious. Obviously, this also breaks combat engagement.