…As with other words I’ve seen people claim come from testicles, it really looks like it’s the other way around. Latin testis primarily means witness, apparently from an Indo-European root *trito-sth2- meaning a third standing by; then they referred to their testicles as witnesses, the same way Greeks called theirs parastatēs.
It’s tough to know why they called the body part that, and Romans swearing by them might be an explanation, but looking into it there doesn’t seem to be any evidence they did that either. Sorry.
