horse•backPronunciation: (hôrs'bak"), [key]
the back of a horse.
a low, natural ridge of sand or gravel; an esker. Cf. hogback.
on horseback: to ride horseback.
made or given in a casual or speculative way; approximate or offhand: a horseback estimate on the construction costs.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.