The Collaborative International Dictionary
On horseback
Horseback \Horse"back`\, n.
The back of a horse.
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An extended ridge of sand, gravel, and boulders, in a half-stratified condition.
--Agassiz.On horseback, on the back of a horse; mounted or riding on a horse or horses; in the saddle.
The long journey was to be performed on horseback.
--Prescott.