The word "万一" is formed through a conceptual metaphor based on numerical scale. It literally means "ten thousand" (万) and "one" (一), combining to create the idea of "one in ten thousand". This construction visualizes probability, where the vast, round number "ten thousand" represents the totality of normal or expected situations, and the singular "one" represents the single, remote exception or unforeseen possibility within that set. Thus, its formation encapsulates the notion of a minuscule chance or a remote contingency by framing it as a stark numerical ratio.