Source : Webster's Revised Unabridged Dictionary (1913)

Bareback \Bare"back`\, adv.
   On the bare back of a horse, without using a saddle; as, to
   ride bareback.

Source : WordNet®

bareback
     adj : riding without a saddle; "a bareback rider" [syn: {barebacked}]

bareback
     adv : without a saddle; "she prefers to ride her horse bareback"
           [syn: {barebacked}]