An idiom meaning someone is helplessly at the mercy of fate or someone else's decision, like a fish on a chopping board.
手術の日が決まり、患者はまな板の上の鯉のように医師の判断に委ねられた。
The surgery date was set, and the patient, like a fish on a chopping board, was left to the surgeon's judgment.
交渉が決着するまでは、彼らはまな板の上の鯉であると感じていた。
Until the negotiations were settled, they felt like fish on a chopping board.
大きな会社の合併に巻き込まれて、小さな支店はまな板の上の鯉の立場になった。
Caught up in a big company's merger, the small branch found itself like a fish on the chopping board.