tenacity (n.)

early 15c., tenacite, "quality of holding firmly," from Old French ténacité (14c.) and directly from Latin tenacitas "an act of holding fast," from tenax (genitive tenacis) "holding fast, gripping, clingy; firm, steadfast," from tenere "to hold" (from PIE root *ten- "to stretch").  The PIE root produced two Latin verbs, tenere "to hold, grasp," and tendere "to stretch" (as in tend (v.1)), which perhaps is from an inflected form in the PIE verb. Both Latin verbs have past participle tentus.