If a body is lissom, it moves in a light, easy, graceful way — able to bend, like a dancer or a cat, not stiff. Though actually the heart of the word is not merely “skinny.” A very thin person who is awkward is not lissom; the compliment is about how the body yields and moves. You will also see lissome, with an extra e — that is the same adjective, not a different one, and in American writing it is the more common spelling. The word is lithe plus -some, and lithesome is its twin. You meet it in stories and poems more than in the supermarket. It is a literary, slightly old-fashioned adjective; at school people would more often say slim, flexible, or graceful.