Horse"back` (?), n.
1.
The back of a horse.
2.
An extended ridge of sand, gravel, and bowlders, 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.
© Webster 1913.