sitar is borrowed from Hindi-Urdu, from Persian ستار sitār, composed of sih "three" and tār "string". sih is from Proto-Indo-European *trei- "three". I'd always assumed that sitar was related to guitar and zither in some way; apparently not.
In Italic, *trei- combined with *steh₂- "stand" to form the compound *tri-stis or *trito-stis meaning "third person standing by", that is, "witness". This became Latin testis "witness". This gives us words like testimony, testify, contest, and maybe also testicle, from Latin testiculus, a diminutive of testis. How the meaning of Latin testis changed from "witness" to "male reproductive gland" is unclear. One hypothesis is that Romans would put one hand over their testicles when giving testimony, so the word came to be associated with the body part. Skeat says the testis was "probably considered as a witness of manhood". However Walde considers a connection between testis "witness" and testis "testicle" unlikely.