The story
From the name of a legendary knot tied to a pole near the temple of Zeus in Gordion. It was prophesied that whoever loosed the knot would become ruler of all Asia. Alexander the Great solved the puzzle by slicing through the knot and took it as a sign of Zeus' favor. He then proceeded to conquer much of the known world at that time.
Etymology from Wiktionary, CC BY-SA 4.0.