A toss-up means the results have not been decided.
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.
1) Do you think they'll make it one time?
Answer: "I really don't know. It's a toss-up.
2) It is a tossup whether we will win or lose.
3) When that day's sun set it was a tossup which side would consider the encounter the greater disaster.
4) The economy is more of a tossup, and the president has a slight edge on taxes.