Where does “τρίγωνον” come from?

τρίγωνον (Ancient Greek) comes from Ancient Greek τρίγωνος, from Ancient Greek γωνία, from Ancient Greek γόνυ, from Proto-Hellenic gónu, from Proto-Indo-European ǵónu — knee.

τρίγωνον (Ancient Greek): triangle; one of the Athenian lawcourts; the...