sinew

Concise Oxford English Dictionary © 2008 Oxford University Press:
sinew/ˈsɪnjuː/

▶noun

• 1 a piece of tough fibrous tissue uniting muscle to bone; a tendon or ligament.

• 2 (sinews) the parts of a structure or system that give it strength or bind it together.

– derivatives
sinewed adjective,
sinewless adjective,
sinewy adjective.

– origin OE sin(e)we ‘tendon’, of Gmc origin.