An Etymological Dictionary of Astronomy and Astrophysics

Stretched tight, as a cord, fiber, etc.; drawn taut; rigid.

Etymology (EN): From L. tensus, p.p. of tendere “to stretch,” → tension.

Etymology (PE): Tanu “stretched, strained,” from tan + -u suffix of excess. The first element tan, from tanidan
“to spin, twist, weave” (Mid.Pers. tanitan; Av. tan- to stretch, extend;" cf. Skt. tan- to spin, stretch;" tanoti “stretches,” tantram “loom;” Gk. teinein “to stretch, pull tight;” L. tendere “to stretch;”
PIE base *ten- “to stretch”), Pers. târ “string,” tân “thread,” tur “fishing net, net, snare,” and tâl “thread” (Borujerdi dialect) belong to this family; variants tanta “cobweb,” tanadu, tafen, kartané, kârtané, kâtené,
Pashtu tanistah “cobweb;” cf. Skt. tantu- “cobweb, thread, string”).