The Collaborative International Dictionary of English v.0.48:

A cheval \A` che*val"\ [F., lit., on horseback.]
   Astride; with a part on each side; -- used specif. in
   designating the position of an army with the wings separated
   by some line of demarcation, as a river or road.
   [1913 Webster]

         A position [`a] cheval on a river is not one which a
         general willingly assumes.               --Swinton.
   [Webster 1913 Suppl.]