Definition of "leanback"

I was placed in a saddle and a leanback was made; a bent twig or sapling inclined at an angle of 50° was attached to the rings at the crupper of the saddle, and I was securely attached to it by a rope.

1867, United States. Surgeon-General's Office, Circular, page 14

On the each end was a leanback made of willow rods tied together in a tapered trapezoidal form set at an angle. Covers of buffalo hide with the hair on and quillwork designs were draped over the leanback and bed surface.

2013, Jeffrey D. Anderson, Arapaho Women's Quillwork: Motion, Life, and Creativity, page 61

Sleeping rolls or mattresses of animal skins were arranged around the circular edges near the buckskin walls, and there were buffalo hide seats called leanbacks randomly placed near the fire pit.

2014, M. B. Tosi, The Thundering Path of Spirit, page 10