One theory is that the word "testify" was derived from the ancient Roman custom of men holding their testicles with their right hands before giving testimony in court.

And why did the Romans have to hold their balls before they could testify in court? It was so that eunuchs and women were excluded.

We should say that etymologists aren't unified on this: some say that the origin of testify came from the latin "testis" which means "third person standing by" or "witness." But that's boring. (Source: American Heirtage Dictionary of the English Language)