Why is a penis called a penis? Most names of our genitals arise from other languages. And once you know the origins of the names for these body parts, you'll discover how sanitized modern language is when it comes to naming sex organs.
Of Tails and Testicles
Most all of our modern medical names for genitals, slang withstanding, are named in the fashion of the rest of our body parts – the names are simply taken from Greek or Latin.
Penis is derived from the Latin word for tail, popping up in Cicero's ad Familiares. By that time, the usage of the word primarily lent itself to this part of male genitalia. The Greek translation of phallus is swell, and the word later came to describe items of penile nature in Latin.
Testicle (and its plural, testes), has a little more unusual etymology. The term testicle likely evolved from the Latin word testis, a term for someone that witnesses or gives testimony in a legal setting.
Clitoris has its origin, you can probably guess if you are a fan of ancient languages, in Greek. Clitoris carries with it connotations for the Greek word for key, kleis. There may be an attribution arising via the verb kleiein , meaning "to close" or "shut", as well. English use of the words vulva and vagina come from identical terms in Latin meaning "wrapper" and "scabbard", respectively.
Finally, ejaculate arises from the Latin word eiaculor, suggesting hurling and shooting. When you look at the original context of the names for these sex organs, they are much more intense and descriptive than modern names.
By using these proper Latin and Greek derivatives, our language has become rather sanitized, but, at the same time given way to vulgarities arising from the use modern names for our sex organs. The Latin derivative of ejaculate — hurling — is far more descriptive, but I would have more trouble running around in public and screaming ejaculate than using its more descriptive etymological grandparent.
Don't Eat that Pizzle
The penis (or tail, if you will…) is the recipient of some of the more inventive mainstream names through the centuries. 14th Century English men and women used the euphemism "yard" for the organ. We've unfortunately lost that usage over time, although it might make for an interesting term to bring back.
Pizzle also pops up in old English via German and Flemish as a term for the penis, particularly when describing the fibrous parts of the organ. Our ancestors attached the pizzles of bulls to the tips of whips and dried them to make chew toys for dogs. Chinese Olympians ate Scottish deer pizzles (either in a stew or as a protein-like powder) in preparation for the 2008 Summer Olympics. If you every find yourself drinking dried and ground up penis to get the upper hand, you can at least giggle when you think of the word pizzle.
Top image is via mararie/Flickr, and shows a typical ornament that would have adorned a house in Pompeii. Additional image is via Gergley Vass/CC and features a a dried bull penis (a pizzle) chopped up to make chew treats for pets.