Late 19th century; earliest use found in Ray Lankester (1847–1929), zoologist. From procto- + ancient Greek ὁδαῖος that is on or by the road (in ancient Greek only in ὁδαῖα freight, merchandise; in Byzantine Greek designating the god Hermes, to whom statues were set up by the roadside; from ὁδός way + -αῖος, suffix forming adjectives) + -um, after Latin nouns in -um.