東西不能用
這一支就是字面：不能被 operate（操作）的東西。
adjective · C1
Unable to be used or to work — said about a machine, a vehicle, a building, or a system that has been damaged or has broken down so badly that it cannot do its job.
無法運作的
核心畫面
One way to hold this word: a thing you cannot work on or work with. Put a hand, a tool or a knife to it and nothing can be done — the machine will not run, and the knife cannot go in. Both uses report the same closed door, one in a workshop and one in an operating room.
可以想成一樣「動不了工」的東西。手伸過去、工具拿起來，卻什麼也做不了：機器轉不動，刀也下不去。兩個用法其實是同一扇關…更多收起
可以想成一樣「動不了工」的東西。手伸過去、工具拿起來，卻什麼也做不了：機器轉不動，刀也下不去。兩個用法其實是同一扇關上的門，一扇在工廠裡，一扇在手術房裡。From the word’s history
From in- + operable, where operable goes back to Latin operabilis, from the idea of work. Both English senses sit on that one root: to operate a machine and to operate on a patient are the same word doing work on something.
拆開來是 in-（不）加 operable，而 operable 上溯到拉丁文 operābilis，字根講的是「工作」。英文的 operate 同時用在「操作機器」和「開刀」，所以加上 in- 之後，兩個意思自然落在同一個根上：無法在上面動工。
Wiktionary etymology tree supplied in packet (kaikki:3f6f69a190036ae4:1281810181:10702)
Two branches
刀下不去
把「動工」的對象從機器換成人體，operate 就變成「開刀」，同一個否定詞跟著搬過去。
adjective · C1
Used about an illness or a growth in the body: doctors cannot treat it by operating on it. It says which treatment is closed off, not that nothing at all can be done.
無法開刀的