is one of the oldest and most enduring Roman legal adage
Contrary to what you might think, it does not have anything to do with the virility of cryptorchid citizen.
It means: "A single witness is as good as none" (or "one witness, no witness" in a more literal fashion).
Beyond its role as one of the fundamental precept of most modern countries legal system, it is also used in other contexts (such as historical research) to assert the necessity of crosschecking a finding through several different sources before stating its accuracy.
And if you must know, there is a reason why the Latin word for "witness" ("testis") looks so much like another modern word:
in ancient Rome it was required for a witness, or anybody else taking oath, to place a hand on their genitals as a pledge of their good faith. Obviously, there was a very misogynistic aspect to it, in that only men were given legal recognition through their right to take such oath. This explains the etymology for words like "testament" or "testimony".
Well, actually it became a huge controvery... Gritchka suggested that basically, this whole "swearing on somebody's balls" theory was a bunch of bollocks and could not be sustained etymologically speaking. And he has a valid case. Read his writing for yourself.
Although he raises an interesting point, I keep considering this rather established etymology much more credible than the "testus/box" one.