The American Heritage Dictionary entry: TESTIS (2024)

[Latin, witness, testis; see TESTIFY.]

Word History: The resemblance between testimony, testify, testis, and testicl* shows an etymological relationship, but linguists are not agreed on precisely how English testis came to have its current meaning. The Latin testis originally meant "witness," and etymologically means "third (person) standing by": the te- part comes from an older tri-, a combining form of the word for "three," and -stis is a noun derived from the Indo-European root stā- meaning "stand." How this also came to refer to the body part(s) is disputed. An old theory has it that the Romans placed their right hands on their testicl*s and swore by them before giving testimony in court. Another theory says that the sense of testicl* in Latin testis is due to a calque, or loan translation, from Greek. The Greek noun parastatēs means "defender (in law), supporter" (para- "by, alongside," as in paramilitary and -statēs from histanai, "to stand"). In the dual number, used in many languages for naturally occurring, contrasting, or complementary pairs such as hands, eyes, and ears, parastatēs had the technical medical sense "testicl*s," that is "two glands side by side." The Romans simply took this sense of parastatēs and added it to testis, the Latin word for legal supporter, witness.