The Latin word for a witness was testis, which is an Indo-European word for the number three. That was because the Romans regarded a witness as a trusted third party, one who stands aside from the dispute and can tell how things really are. The Romans did also use the word testis in a figurative way to mean testicle. The idea seems to have been that a testicle was a witness to a man’s virility.
Men held each others testicles as a sign of truthfulness while bearing witness in a public forum. The Romans found a word to describe this practice but didn’t invent the practice itself. Other primates had already been doing this for millions of years. This behavior has nothing to do with sex but it’s a social ritual that primatologists call a “greeting.”
