An Etymological Dictionary of Astronomy and Astrophysics

A polygon having eight angles and eight sides.

Etymology (EN): From L. octagonos, from Gk. oktagononos “eight-angled,” from okta-, → octa-, oct- “eight,” + gonia “angle,” related to gony “knee,”
L. genu “knee,” cuneus “a wedge;” Av. žnu- “knee;” Mod.Pers. zânu “knee,” Skt. janu- “knee,” kona- “angle, corner;” PIE base *g(e)neu-, see below.

Etymology (PE): Haštbar “eight-sided,” from hašt “eight,” → octa-, oct- + bar “side; breadth; breast” (Mid.Pers. var “breast;” Av. vouru “wide, broad, extended” (vourucašāni- “looking far”), related to varah- “breast;” cf. Skt. urú- “wide, broad,” úras- “breast;” Gk. eurus “wide, broad;” PIE base uer-, ueru-s“wide, broad”); haštguš, from hašt, → octa-, oct-, + guš “corner, angle,” Mid.Pers. gošak “corner.”