Meaning

noun an awkward and inexperienced youth

originInherited from Middle English grene horn, which is attested for “horn of a recently killed animal” and as the name of a horse. It may also have been used of young horned animals. In all cases “green” refers to the idea of “fresh, young, recent”, taken from plants and used in Middle English for all kinds of things irrespective of their colour, e.g. wounds, leather, fish, cheese (see green cheese).…

Etymology from Wiktionary, CC BY-SA 3.0