sinew

Sinew comes from Middle English synwe or synewe, meaning tendon, ligament, muscle, nerve, or even leaf vein, from Old English sinu. This traces back through Proto-West Germanic *sinu and Proto-Germanic *sinwō to the Proto-Indo-European root *snéh₁wr̥, itself from *(s)neh₁-, meaning "to twist threads, spin, weave."

In Middle English the word covered a wide range of body parts and connective tissue — tendons, ligaments, muscles, nerves, even the veins of a leaf — before narrowing in modern English to the tough, cord-like tissue joining muscle to bone.

Sinew is a doublet of nerve and neuron, both ultimately from the same Proto-Indo-European root — Latin nervus and Ancient Greek neûron carried the same original sense of a fibrous, twisted cord.