horseback

n. 1. The back of a horse. [ 1913 Webster ]

2. An extended ridge of sand, gravel, and boulders, in a half-stratified condition. Agassiz. [ 1913 Webster ]

On horseback, on the back of a horse; mounted or riding on a horse or horses; in the saddle. [ 1913 Webster ] The long journey was to be performed on horseback. Prescott. [ 1913 Webster ]