On or by means of the back of a horse; while mounted on a horse.
Combination of 'a-' (on) and 'horseback' (the back of a horse). This archaic construction emphasizes the location or position of the rider.
Before cars, 'ahorseback' was just a normal way to describe traveling—now it's almost never used, but you'll see it in old novels and adventure stories where it sounded completely ordinary.
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