Testimony \TES-tuh-moh-nee\ or \TES-tuh-muh-nee\ , noun;
1. Evidence in support of a fact or statement; proof
2. Law: The statement or declaration of a witness under oath or affirmation, usually in court
3. Open declaration or profesion, as of faith
I saw a "useless knowledge" factoid yesterday that said testimony came from a practice of covering one's testicles as a symbolic gesture of telling the truth. Kind of like swearing on a bible, except they're balls.
This practice may or may not have happened, but testimony and testes are linguistically connected. Dating to the 1300's, testimony comes from Latin testimonium, with the root testis meaning "witness". Testes is the plural of testis, which came directly from Latin testis ("testicle") in the early 18th century. It is considered a special application of testis ("witness") because the presence of testicles 'bears witness' to one's virility.