A sinewy person has a thin body and strong muscles you can see — not soft, not heavy, but strong in a lean way. Writers use it for athletes, dancers, farm workers, older people who are still tough. A sinew is the strong cord that joins muscle to bone; sinewy means “full of that kind of strength.” Though actually, the word has other lives. Meat that is sinewy is full of those tough cords and hard to chew — not a compliment on a plate. And people call writing or an argument sinewy when it is lean and strong, with no extra fat. For a body, it is usually praise; for a steak, it is a complaint. Skinny only says there is little flesh. Sinewy says the flesh that is there is strong.