The phrase "inexhaustible source" means that something is unlimited or endless and will never run out. It refers to a resource or supply that can be continuously used without being depleted or diminished. Full definition
«Words are, in my not so humble opinion, our most inexhaustible source of magic, capable of both inflicting injury and remedying it.» (gogrounded.com)
There's a brief respite that takes place in something like an afterlife (one vastly less infuriating than The Tree of Life's, incidentally), in which a character urges that «words are our most inexhaustible source of magic.» (austinchronicle.com)