Source : Webster's Revised Unabridged Dictionary (1913)

Astraddle \A*strad"dle\, adv. [Pref. a- + straddle.]
   In a straddling position; astride; bestriding; as, to sit
   astraddle a horse.

Source : WordNet®

astraddle
     adv : with one leg on each side; "she sat astride the chair" [syn:
            {astride}]