horse's back
Literal meaning: the back of a horse. Used in contexts of riding or describing the animal.
He rode on the horse's back across the grassland.
mountain ridge; arête; road along a ridge
Figurative use: a narrow mountain ridge or a road running along such a ridge, likened to the shape of a horse's back. Often used in geographical descriptions or hiking contexts.
The trail follows a ridge like a horse's back.
Please watch your step when walking along the narrow ridge.
Compound of 馬 (uma, 'horse') + の (possessive particle) + 背 (se, 'back'). The figurative sense derives from the resemblance of a narrow mountain ridge to the curved back of a horse.