Something or someone that is supple bends and moves easily, like a contortionist at a circus sideshow. If you can wrap your legs around your neck, you most likely have a supple body.
Supple is often used as an adjective describing a body — such as a dancer’s; a fabric — such as soft leather; or a sound — such as an accomplished singer’s voice. This makes for a strange supple family, but all of its members can move with ease and flexibility. More idiomatically, supple can also be used to describe a mind. In this sense, it does not mean “easily brainwashed,” but has a more positive association with the open-mindedness of free-thinkers.