The spelling of the word "heptagonus" may seem confusing, but it follows the rules of Latin and Greek suffixes. The word is comprised of "hepta" meaning seven and "gon" meaning angle or corner, both of which come from Greek. The suffix "-us" is of Latin origin and signifies a noun or adjective. Therefore, the pronunciation of "heptagonus" can be transcribed using IPA as /hɛptəɡonəs/. This word refers to a polygon with seven sides and angles.
Heptagon, a seven-cornered figure.
A practical medical dictionary. By Stedman, Thomas Lathrop. Published 1920.