The Collaborative International Dictionary of English v.0.48:

Bestride \Be*stride"\, v. t. [imp. Bestrode, (Obs. or R.)
   Bestrid; p. p. Bestridden, Bestrid, Bestrode; p. pr.
   & vb. n. Bestriding.] [AS. bestr[imac]dan; pref. be- +
   str[imac]dan to stride.]
   1. To stand or sit with anything between the legs, or with
      the legs astride; to stand over
      [1913 Webster]

            That horse that thou so often hast bestrid. --Shak.
      [1913 Webster]

            Why, man, he doth bestride the narrow world
            Like a Colossus.                      --Shak.
      [1913 Webster]

   2. To step over; to stride over or across; as, to bestride a
      threshold.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bestride
    v 1: get up on the back of; "mount a horse" [syn: hop on,
         mount, mount up, get on, jump on, climb on,
         bestride] [ant: get off, hop out]