14世紀晚期，“不可摧毀的，無法鬆動的”，也比喻地用於問題等，“無法解決或解釋的”，源自古法語 insoluble 或直接源自拉丁語 insolubilis “無法鬆動的”，來自 in- “不”（見 in-（1））和 solubilis “可鬆動的”（見 soluble）。
It was a tacit conviction of the learned during the Middle Ages that no such thing as an insoluble question existed. There might be matters that presented serious difficulties, but if you could lay them before the right man -- some Arab in Spain, for instance, omniscient by reason of studies into the details of which it was better not to inquire -- he would give you a conclusive answer. The real trouble was only to find your man. [Gertrude Bell, "The Desert and the Sown," 1907]
中世紀學者們心照不宣地認爲，不存在無法解決的問題。可能會有一些問題存在嚴重的困難，但如果你能把它們放在正確的人面前——例如西班牙的某個阿拉伯人，由於研究的細節而無所不知——他會給你一個決定性的答案。真正的麻煩只是找到你的人。[杰特魯德·貝爾，“沙漠與耕種”，1907年]
意思是“無法在液體中溶解”的，始於1713年。
