testis

Testis, testis is third declension, i-stem, genitive plural testium, and it is common gender: the same form does for a male and a female witness, so only the agreeing adjective or participle tells you which. The likeliest etymology is tri-sto, "the third party standing by", the outsider to a dispute between two.

The trap is anatomical. The other testis, "testicle", is the same word, and the schoolroom story about Romans swearing oaths on them is folk etymology; the sober explanation is that it "bears witness" to manhood. Either way, a plural testes needs a moment's thought before you commit to a translation. The legal vocabulary around it is worth having: testor, "to bear witness, to make a will", testamentum, testimonium, contestor. English testify, testimony, testament, attest, protest, contest and detest all sit on this one noun.