To say probably not means to be indesicive where you're not fully saying "yes" or "no" and you're not too sure about a certain situation unless it's presented to you in the right manner.
a bull shit part of math that doesn't matter in the real world because no matter how small the odds are there's always the chance of that thing happening
A: What if a bomb goes off? (IDK I couldn't think of anything bite me)
B: What's the actual probability of that happening?
A: Who gives a fuck