Have you ever seen people who can bend so easily, they can touch their heels to the back of their heads? Those people are, in a word, lithe.
Lithe comes to us from Old English and originally meant "mild, meek." As a meek person bends to the will of others, the meaning of lithe has broadened to flexible and even graceful. Think of a dancer or the ease of a sleek cat when you think of lithe. You can use it to describe a person or the way someone moves.