The word testify comes from the Latin word for testicles.
In ancient Rome, when taking an oath of allegiance to each other, the two men would exchangeably hold each others testicles. Actually, the Latin word for witness is testis. Again, when testifying in a court as a witness, one would do so while squeezing his testicles.
The question then is, why holding of each others testicles .
The act was actually not invented by humans, but other primates, baboons. Two male baboons who cooperate with each other by forming aggressive alliances against other baboons frequently fondle each other's genitalia. In a nutshell, when two male baboons, let's call them Njoro and Odhis, fondle each other's genitalia, they both take a huge risk. By letting Odhis fondle his testicles, Njoro shows a great deal of trust in Odhis's good intentions. Odhis could quickly and easily terminate Njoro's reproductive career for good by ripping his testicles off. In other words, testing the strength of the bond involves behaving in ways that are costly, risky or otherwise detrimental to the partner.
By the way, this is called "The Handicap Principle"
Anyway in, when a man made a solemn oath in ancient times, he placed over his testicles, his most precious part, the source of his seed, indicating by his action that he believed he would become impotent if he lied.