Something you push at that pushes back. An order, a medicine, a fire, a signal — you apply it, and the thing does not take it. The force comes back off, and the thing stays exactly as it was.
Latin refractarius 'obstinate', from refringere 'to break up, break back'. The picture of force being broken back off something sits under every modern use.
拉丁文 refringere 是「打斷、折回」，形容詞 refractarius 指「硬脾氣的」。英文原本寫成 refractary，後來比照其他 -ory 結尾的形容詞改了拼法。「力量被折回來」這個畫面，正好串起今天所有的用法。
Wiktionary entry for refractory (kaikki extract, source_ref kaikki:3f6f69a190036ae4:961142316:26832): 'From Latin refractārius (obstinate), from refrāctus, past participle of refringere (to break up).'
Three branches
人不服管
被折回來的是命令。話說出口了，對方沒動，而且是故意沒動。這是這個字最早、也最一般的用法。
adjective · C2
Refusing on purpose to obey a person in charge, and staying hard to manage or deal with. It is a formal word, used about people, animals, and sometimes about things that will not do what you want.
不服管教的
治不好、叫不動
這裡被折回來的不是命令，是藥效和訊號。對象不是故意的，但結果一樣：加上去的東西沒有進去。從「人不聽話」走到「身體不反應」，中間只換了施力的那一方。
adjective · C2
Used about an illness, a symptom or a patient that does not get better with the usual treatment, so doctors have to look for another way.
難治的
燒不壞
這一組被折回來的是熱。火加上去，材料不變形、不熔化，熱被擋在外面。這是 LDOCE 沒有收、但今天工業界最常用的一支。
adjective · C2
Able to stay solid and keep its shape at very high temperatures, so it can be used to line a furnace, a kiln or an oven.
耐火的