I have a pet peeve. Over the years, I've heard many people call or spell the testis a "testi" or "teste", perhaps from seeing testis or testes and removing the "s" thinking that it's plural. Since it's a latin word, the singular form is testis and the plural is testes.
So where does testis come from? Well, there is a commonality between the male gonad and a court of law. No, this is not a lawyer joke. Let me explain...When you are a witness in court you testify and in a man's scrotum are testicles or testes.
They share the same root word, testis, which means a witness.
Some related words from Wiktionary are:
How does this word relate to a the male anatomy? This root word was applied to the male gonad because it is a "witnesses for a man's virility".
It's funny how I've been involved in urology for many years and didn't know all these details about the word until now!