a cheval
Webster's Unabridged Dictionary:
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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.
A position à cheval on a river is not one which a general willingly assumes.Swinton.
[Webster 1913 Suppl.]
