expression
riding so skillfully that man and horse become one
Idiomatic expression describing a rider so accomplished that the distinction between rider and horse disappears. Often used in martial arts, horsemanship, or metaphorically for any masterful unity of person and tool.
彼の乗馬はまさに鞍上人なく鞍下馬なしの境地だ。
His riding truly reaches the state where man and horse are one.
武道の達人は、鞍上人なく鞍下馬なしのごとく武器を操る。
A martial arts master wields a weapon as if rider and horse were one.
A more common four-character compound meaning 'unity of rider and horse'. 鞍上人なく鞍下馬なし is a longer, more literary idiomatic expression with the same core meaning.
A classical Japanese idiom literally meaning 'on the saddle there is no person, under the saddle there is no horse', describing perfect unity between rider and mount. The exact origin is uncertain, but it reflects traditional equestrian and martial ideals.