A horse used by a bareback rider or acrobat; a bareback rider. (1923 —) . C.B.Cochran A 'rosin-back' is a ring-horse used by bareback riders....Rosin is rubbed into the horse's back to help the rider get a firm footing as he jumps from the ring on to the horse (1945).
[From rosin resin, with which the horse's back was rubbed for a firmer seat.]