From WordNet (r) 2.0 (August 2003) [wn]:
horseback
n 1: the back of a horse
2: a narrow ridge of hills [syn: hogback]
adv : on the back of a horse; "he rode horseback to town";
"managed to escape ahorse"; "policeman patrolled the
streets ahorseback" [syn: ahorse, ahorseback]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Horseback \Horse"back`\, n.
1. The back of a horse.
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2. An extended ridge of sand, gravel, and boulders, in a
half-stratified condition. --Agassiz.
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On horseback, on the back of a horse; mounted or riding on
a horse or horses; in the saddle.
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The long journey was to be performed on horseback.
--Prescott.
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