The Collaborative International Dictionary of English v.0.48:

Astride \A*stride"\, adv. [Pref. a- + stride.]
   With one leg on each side, as a man when on horseback; with
   the legs stretched wide apart; astraddle.
   [1913 Webster]

         Placed astride upon the bars of the palisade. --Sir W.
                                                  Scott.
   [1913 Webster]

         Glasses with horn bows sat astride on his nose.
                                                  --Longfellow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

astride
    adv 1: with one leg on each side; "she sat astride the chair"
           [syn: astride, astraddle]
    2: with the legs stretched far apart