- eo- prefix used from mid-19c. (first in EOCENE (Cf. Eocene)) in compound words formed by earth-scientists and meaning вЂњcharacterized by the earliest appearance of,вЂќ from Gk. eos вЂњdawn,вЂќ from PIE *aus-, cognate with Mod.Eng. EAST (Cf. east) (q.v.). Piltdown Man, before exposed as a fraud, was known as Eoanthropus.
Etymology dictionary. 2014.