sinew

Old English 'sinu' (also spelled 'seonu') named the fibrous cord joining muscle to bone, and Anglo-Saxon poets reached for it when they needed an image of raw physical force. The word traces back to Proto-Germanic sinwan, which itself descends from a Proto-Indo-European root meaning to bind or twist. The Beowulf poet used it to describe the monster Grendel's arm: the sinews held the grip. By 1000 CE the word was already doing double duty, naming both the anatomical structure and the strength it embodied.

As Middle English absorbed French and Latin vocabulary, 'sinew' held on where the technical term 'tendon' (from Latin tendere, to stretch) eventually competed. The Old Norse cognate 'sin' described the same tissue in Icelandic sagas, and the German 'Sehne' carries the same root into modern usage. English surgeons in the 1600s began preferring 'tendon' in medical writing, but 'sinew' survived in common speech and literary register. John Milton used it in 1667 in Paradise Lost to describe Satan's muscular resolve.

The metaphorical extension happened early. By the 14th century, 'sinews of war' had become a political idiom for money and resources, the connective tissue that holds an army together. Francis Bacon recycled the phrase in his 1612 essay 'Of the True Greatness of Kingdoms,' and it passed into standard English political rhetoric. The same logic drives phrases like 'sinews of society' and 'backbone,' which treat the body's structural elements as metaphors for institutional strength.

Modern anatomy prefers 'tendon' for the muscle-to-bone connection and 'ligament' for bone-to-bone. But 'sinew' still appears in veterinary science, traditional medicine, and anywhere writers want a word that feels older and more bodily than its Latin replacement. It persists in idioms, in poetry, and in the language of physical labor, where the word retains a weight that 'tendon' never quite achieves.