Think of a cloth bag. There is one opening at the top, and the bottom is sewn shut. Anything that goes in has to come back out the same way, because there is nothing at the far end. French named this word exactly that: the bottom of a bag. Every use of cul-de-sac can be laid on that picture — a street you must reverse out of, a plan that stops dead, a pouch in the body with one way in.