1691, uncredited translators, Plutarch’s Morals, Part 2, London: T. Sawbridge et al., p. 224, […] they took her and set her on Ass-back, and led her round about the City,
The way of riding most used in this place is on assback.1766, Tobias Smollett, Travels Through France and Italy, London: R. Baldwin, Volume 1, Letter 5, p. 74:
2 more examples
Riding on assback was accounted a disgrace and a degradation to the Gothic hidalgo,1846, Richard Ford, chapter 7, in Gatherings from Spain,, London: John Murray, page 74:
1994, Robert Kelly, “In Irish America” in Queen of Terrors, Kingston, NY: McPherson, p. 56, […] tune soft as the assback my Savior rode from Jericho to Caerleon.
[…] I should have thought a great chief had something better to do than to play chess either horseback or assback.1850, George Walker, “A Game of Chess with Napoleon”, in Chess and Chess-Players, London: Charles J. Skeet, page 260:
That samurai is surely fast alive In his hunched and silhouetted dozing, assback, In the blur of fog,1970, Mark Perlberg, “Hiroshige”, in The Burning Field,, New York: William Morrow, page 7: