In his article, ‘A “WITNESS” AND A “TESTICLE”? A LINGUISTIC ANALYSIS OF THE LATIN WORD “TESTIS”’ (Carmenta, Sep 26, 2014.), Harvard scholar Larry Myer gives the following philological analysis, based largely on the works of the Princeton classicist, Joshua Katz:

‘Students of Latin are often struck by the fact that the same Latin word testis meant both a “witness” and a “testicle.” In fact, ancient Roman writers, like Plautus, sometimes played with this double meaning. Surprisingly, no scholar had satisfactorily accounted for the origin of this puzzling ambiguity until 1998, when the Princeton Classicist Joshua Katz published his article “Testimonia Ritus Italici: Male Genitalia, Solemn Declarations, and a New Latin Sound Law” in Harvard Studies in Classical Philology.

‘According to Katz (and others before him) the original Indo-European form of testis was trito-sth2-i meaning “a third person standing,” i.e. a third person standing by in order to witness some event (sth2, the second part of the IE form, is related to the Latin word sto, stare to stand). So testis originally meant a “witness.” But how did it come to mean “testicle” as well? In order to answer this question, Katz begins by citing Near Eastern examples of men holding someone’s genitals while they swear an oath. In one famous instance from the Hebrew Bible, Jacob instructs his son Joseph...'