penis (n.)

"the male organ of copulation," 1670s, from French pénis or directly from Latin pēnis "penis," earlier "tail," from PIE *pes-, usually said to be originally "penis" (source also of Sanskrit pasas-, Greek peos, posthe "penis," probably also Old English fæsl "progeny, offspring," Old Norse fösull, German Fasel "young of animals, brood"). But de Vaan writes that "the meaning of pēnītus ['furnished with a tail'] as well as general semantic considerations suggest that the meaning 'tail' is original, and 'penis' metaphorically derived from it." The proper plural is penes. The adjective is penial. In psychological writing, the term penis envy is attested by 1922.