Early 17th century (in an earlier sense). From classical Latin octangulus (adjective) having eight angles (Apuleius, 2nd cent. a.d.) from octō- + angulus. In use as noun, probably after triangle, quadrangle, etc.
Definition of octangle in US English:
octangle
nounˈɒktaŋɡl
A figure with eight angles; an octagon.
Origin
Early 17th century (in an earlier sense). From classical Latin octangulus (adjective) having eight angles (Apuleius, 2nd cent. a.d.) from octō- + angulus. In use as noun, probably after triangle, quadrangle, etc.