Any very difficult problem; insoluble in its own terms.
2.
An intricate knot tied by Gordius, the king of Phrygia, and cut by the sword of Alexander the Great after he heard that whoever undid it would become ruler of Asia.
... etc., at first with very dilute sulphuric acid, and then with ammonia. Afterward we rinsed them with quantities of water and dried them carefully with white linen rags that had been used for no other purpose; and finally we plunged them again into very clean water. We thus cut the Gordian knot, and were on ... — Scientific American Supplement, No. 401, September 8, 1883 • Various
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