Testis means both testicle and witness.
According to the etymological dictionary, this is presumably because testicles bear witness to a man’s virility.
P.S. This time, the author of this blog asks readers to refrain from composing limericks unless absolutely necessary.
Eric Koch’s new book, New Beginnings, was launched on September 20. The book is available from 
The very thought is unscrotable!
Testify and testimony are cognates. It is said that in ancient times taking an oath involved touching the testes. See Genesis 24; 2-3:
“And Abraham said to his servant, the elder of his house that ruled over all that he had: ‘Put, I pray thee, thy hand under my thigh. And I will make the swear by The Lord …'”
As a matter of interest, in an early feminine dictionary, perhaps the first, there was an entry for evidence given by a woman “overimony”.
“in ancient times taking an oath involve touching the testes” — My go-to source for etymologies, Partridge’s Origins, defends the ” witness to virility” derivation, and doesn’t mention the “touching” theory. But the biblical reference (which I was unaware of: thanks, HK!) seems to give the latter some weight. For what it’s worth, testicle is a diminutive of testis.