point beyond which a horse cannot travel
Historical term for a spot on a mountain pass where the trail becomes too steep or rough for horses, often marking a trailhead or turnaround point. Now mostly encountered in place names or historical descriptions.
On old highways, there were many places called 'umagaeshi' where horses could go no further.
From the umagaeshi up ahead, you have no choice but to continue on foot.
Compound of 馬 (uma, 'horse') and 返し (kaeshi, 'turning back'), literally 'horse turn-back'. The exact historical derivation is uncertain, but the term clearly describes a point where horses must be turned back.