This isn't really the etymology but it is the practical implication: polygon means "many 1-cells". All 1-cells are line segments. Therefore, a polygon is made of line segments. Why is a single line segment called a digon and not a monogon? Doesn't "digon" imply that there are two line segments connected at their two vertices? (This is degenerate in Euclidean geometry, they overlap, but in ellipsoidal geometry you can have either a lune or an "equatorial digon," as well as an "equatorial monogon.")
I've also seen "dyad" but I feel this alludes more to the idea of the 0-sphere, a disjoint point pair in 1D. "Ditelon" (polytelon=1D shape) seems like the most accurate name, but it requires you first qualify the idea of the telon, the 0-cell (point), as a constructing object, which is quite niche and limited in use. So what is a good and accurate name for a line segment that isn't the unwieldy "line segment," and why was it ever called a "digon" in the first place?