penis (n.) Look up penis at Dictionary.com
1670s, perhaps from French pénis or directly from Latin penis "penis," earlier "tail," from PIE *pes- "penis" (cognates: 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.