When something is a "toss-up", we are not sure what the result will be. The analogy is to tossing a coin as a way of making a decision. When "flipping" (or "tossing") a coin there is a 50 percent chance that the toss will end up one way and a 50 percent chance that the toss will end up another way. When something is very close and we can't tell if it should be one way or the other, we say that it is a toss-up.