The Collaborative International Dictionary
Ahorseback \A*horse"back\, adv. On horseback.
Two suspicious fellows ahorseback.
--Smollet.
Wiktionary
adv. On the back of a horse; horseback#Adverb.
WordNet
Usage examples of "ahorseback".
Vail, who was getting a bit of a belly on him now that his days were spent largely riding a chair instead of being out ahorseback in active pursuit of wanted felons, paced the length of the room a few times in silence before taking his own much more comfortable seat at the desk.
Now it was in the cool of the evening two days after the Battle on the Ridge, that the men, both freemen and thralls, had been disporting themselves in the plain ground without the Burg in casting the spear and putting the stone, and running races a-foot and ahorseback, and now close on sunset three young men, two of the Laxings and one of the Shieldings, and a grey old thrall of that same House, were shooting a match with the bow, driving their shafts at a rushen roundel hung on a pole which the old thrall had dight.