mountain ridge
Refers to the ridge or spine of a mountain. A straightforward compound of 山 (mountain) and 背 (back/spine).
登山道は山の背に沿って続いている。
The trail continues along the mountain ridge.
Compound of 山 (mountain) and 背 (back/spine), literally 'mountain's back'. The exact historical derivation is uncertain, but the meaning is transparent.