1560s, tied by Gordius, king of Phrygia in Asia Minor, who predicted the one to loosen it would rule Asia. Instead, Alexander the Great cut the Gordian knot with his sword; hence the extended sense (1570s in English) "solve a difficult problem in a quick, dramatic way."
A complex knot tied by a Greek king. According to legend, whoever loosed it would rule all Asia. Alexander the Great, according to some accounts, undid the Gordian knot by cutting through it with his sword.
Note: By extension, to “cut the Gordian knot” is to solve quickly any very complex problem or to get to the heart of a problem.