Of American-English origin, the noun rosinback denotes, in circus slang, a horse used by a bareback rider or acrobat.

The British impresario and theatrical producer Charles Blake Cochran (1872-1951) explained the origin of this noun in Showman Looks On (London: J. M. Dent & Sons Ltd., 1945):

The circus is a kingdom of its own with its own hierarchy, traditions, and language. For instance, a ‘rosin-back’ is a ring-horse used by bareback riders. The origin of the term is obvious; rosin is rubbed into the horse’s back to help the rider to get a firm footing as he jumps from the ring on to the horse.

The noun rosin denotes a kind of resin obtained as a residue after the distillation of crude turpentine oleoresin, or of naphtha extract from pine stumps. Rosin is, in particular, rubbed on the bow hair of violins and similar stringed instruments, and applied in powdered form to shoes, gloves, etc., by dancers and sports competitors to prevent slipping.

The earliest occurrences of the noun rosinback that I have found are as follows, in chronological order:

1-: From an interview of William Smith, in charge of “the horses that have been appearing at the Barnum & Bailey big circus in the Madison Square Garden”, published in The New-York Times (New York City, New York) of Friday 24^th April 1896:

“There are sixteen of the rosin-backs, as we call them—the ring horses for bareback riding. They are most of them Normans. The principal thing in a ring horse is a big, square body, short between joints.”

2-: From the account of a visit to the Ringling Brothers’ Circus, published in the St. Louis Post-Dispatch (St. Louis, Missouri) of Friday 7^th May 1897:

In the next tent were the ring horses—“Rosin-backs,” in circus parlance—the racers and the menage horses. The ring horses are of the same quality as the draughts. It takes about three years to train one for bareback trick-riding, and even then he may get stubborn and have to be relegated to the wagon brigade. The rosin on their backs to keep the riders from slipping is removed once a week with pipe clay. It is an all-day job for an expert hostler.

3-: From the account of a visit to the combined Forepaugh-Sells Brothers’ Circus, published in the Anderson Intelligencer (Anderson, South Carolina) of Wednesday 16^th November 1898:

“Come in here and we will take a look at the rosin backs,” said Schumate.
Then he explained to me that a rosin back horse was one used in the ring for bareback riding. They are the broad-back horses, and the finest of rosin is sprinkled into their hair so that the feet of the rider will not slip. The rosin is taken out once a week, generally on Sunday morning.
Pipe clay is rubbed over the horse’s back, and then this with the rosin is washed out with warm water. If this is not done the back of the horse will blister.

4-: From the account of the arrival of the Ringling Brothers’ Circus at Tattersall’s, published in The Daily Inter Ocean (Chicago, Illinois) of Monday 10^th April 1899:

A busy force of circus men arranged things inside the huge building. The 500 horses were provided for in five tiers of stalls arranged for them in the south end of the building, climbing up the runways to their respective floors with no kicks on account of the lack of elevators. They swished their tails with evident enjoyment and reared about in their stalls as much as their stout halters would allow. The band horses, even, were so far affected as to go through their steps. The horses were assigned stalls in due order of precedence. The barebacks, or rosin-backs, were given quarters on the second floor, the racers occupied select quarters on the third floor; the ponies, many of them Shetlands, looked out over the three rings on the ground below from the fourth floor, and the draft horses were obliged to climb to the fifth.