Horseback is the back of a horse, as a place to ride. On horseback, a horseback ride, arrive on horseback.
The word appears almost only in the phrase on horseback, which is how English says mounted. A horseback opinion is an American phrase for a rough judgement given on the spot, from the idea of an answer given without dismounting.
1nounhorsebackThe back of a horse, as a place to ride.
“Police officers on horseback patrolled the crowded park.”
horseback is a place.
Horse plus back, recorded in Middle English and used from the first mainly in the phrase on horseback.
Middle English
horsebacke
English, today
horseback
From Middle English horsebacke, horsebak, horsbak, equivalent to horse + back.
Etymology from Wiktionary, CC BY-SA