A sinew is a tough, fibrous cord that connects muscle to bone in the body. You might also hear it called a tendon. When you flex your arm or wiggle your fingers, sinews are working beneath your skin, transferring the pulling force from your muscles to move your bones. If you've ever seen raw chicken or beef, those whitish, stringy parts that are hard to cut through are sinews.

The word comes from an old sense of strength and toughness. When someone describes a person as sinewy, they mean the person looks lean and muscular, with visible tendons and defined muscles. A sinewy hand gripping a baseball bat or a rock-climbing hold shows the working strength beneath the skin.

Writers sometimes use sinew more broadly to mean the source of strength in something. The sinews of war might refer to the money, supplies, and resources that keep an army fighting. When someone talks about the sinews of an organization, they mean the essential structures that hold it together and make it work. Like the physical sinews in your body, these are the hidden parts that provide real strength and make action possible.