ahorseback

from WordNet (r) 3.0 (2006)

ahorseback
    adv 1: on the back of a horse; "he rode horseback to town";
           "managed to escape ahorse"; "policeman patrolled the
           streets ahorseback" [syn: {horseback}, {ahorse},
           {ahorseback}]
    adj 1: traveling on horseback; "a file of men ahorseback passed
           by" [syn: {ahorse(p)}, {ahorseback(p)}]

from The Collaborative International Dictionary of English v.0.48

Ahorseback \A*horse"back\, adv.
   On horseback.
   [1913 Webster]

         Two suspicious fellows ahorseback.       --Smollet.
   [1913 Webster]