The Collaborative International Dictionary of English v.0.48:

Penis \Pe"nis\ (p[=e]"n[i^]s), n. [L.] (Anat.)
   The erectile external sexual organ of males, used in
   copulation, and in mammals, also for urination.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

penis
    n 1: the male organ of copulation (`member' is a euphemism)
         [syn: penis, phallus, member]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "penis":
   bag, ballocks, balls, basket, beard, breasts, cervix, clitoris,
   cod, cods, cullions, family jewels, female organs, genitalia,
   genitals, gonads, labia, labia majora, labia minora, lingam, lips,
   male organs, meat, nuts, nymphae, ovary, phallus, private parts,
   privates, privy parts, pubic hair, pudenda, reproductive organs,
   rocks, scrotum, secondary sex characteristic, sex organs, spermary,
   testes, testicles, uterus, vagina, vulva, womb, yoni