Source : Webster's Revised Unabridged Dictionary (1913)

Sidesaddle \Side"sad`dle\, n.
   A saddle for women, in which the rider sits with both feet on
   one side of the animal mounted.

   {Sidesaddle flower} (Bot.), a plant with hollow leaves and
      curiously shaped flowers; -- called also {huntsman's cup}.
      See {Sarracenia}.

Source : WordNet®

sidesaddle
     n : a saddle for a woman; rider sits with both feet on the same
         side of the horse
     adv : on or as if on a sidesaddle; "she rode sidesaddle"