Is it farther or further?
1670s, perhaps from French pénis or directly from Latin penis "penis," earlier "tail," from PIE *pes-/*pesos- "penis" (cf. Sanskrit pasas-, Greek peos, posthe "penis," probably also Old English fæsl "progeny, offspring," Old Norse fösull, German Fasel "young of animals, brood"). The proper plural is penes. The adjective is penial. In psychological writing, penis envy is attested from 1924.
penis pe·nis (pē'nĭs)
n. pl. pe·nis·es or pe·nes (-nēz)
The male organ of copulation and of urinary excretion, formed by three columns of erectile tissue, two arranged laterally on the dorsum and one medianly below; the extremity is formed by an expansion of the corpus spongiosum, covered by a free fold of skin.