sinew: the tough cord inside a body that ties muscle to bone and lets the frame hold together and move
This is the body's inner cabling: the tough, fibrous cords that fasten muscle to bone and let a frame bend, grip, and hold. Hebrew keeps one word for every such cord, where modern medicine separates tendon from ligament and both from nerve. So when a writer names what a body is built of, this word takes its place beside bone (עֶצֶם), flesh (בָּשָׂר), and skin (עוֹר).
Almost every use falls in a scene of a body being assembled or held together. Job says God knit him with bones and sinews. Ezekiel watches dry bones grow sinews first, then flesh, then a covering of skin. When Job's poem admires the bulk of a great beast, it points to the sinews of its thighs, woven tight as rope.
Because a sinew is strong and stiff at once, the word is ready-made for a picture of stubbornness. Isaiah tells a people who will not turn that their neck is an iron sinew: not simply unwilling to bend, but built so that it cannot.
One use hardened into a lasting practice. Genesis explains that Israel does not eat the sinew of the hip, because that is where the man who wrestled Jacob struck him. The verse names one particular cord in one particular place, and the custom it records outlived the story.