"sinew : (anatomy) A cord or tendon of the body. A cord or string, particularly (music) as of a..."

“sinew : (anatomy) A cord or tendon of the body.
A cord or string, particularly (music) as of a musical instrument.
(figuratively) muscular power, muscle; nerve, nervous energy; vigor, vigorous strength.
(figuratively, often in the plural) That which gives strength or in which strength consists; a supporting factor or member; mainstay.”

- https://en.wiktionary.org/wiki/sinew#English

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