The Collaborative International Dictionary of English v.0.48:

Rescussor \Res*cus"sor\ (r?s-k?s"s?r), n. [LL.] (O.Eng.Law)
   One who makes an unlawful rescue; a rescuer. --Burril.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

RESCUSSOR. The party making a rescue, is sometimes so called, but more
properly he is a rescuer.