A testimonial is (the dictionary definition) a statement accounting benefits recieved. Witnesses in court give testimonies about the crimes they saw, and stars give testimonies about how Slim-Fast actually works.

The origin of this word is very interesting. It stems from the Greek 'testicle'. In old Athens courts, where only men could talk, you would swear your testimony was the truth on your manhood, and you would show this by grabbing your crotch, a sacred thing back then.