Similar to an ultimatum, but one where the answer is truly "maybe" to both possibilities. In most cases an ultimaybe is much harder to deal with than an ultimatum due to it's confusing unanswerable nature.
Person A: You should really go to the dance with your girlfriend instead of that really pretty girl you're not actually in a relationship with.
Person B: But... I could be...
Person A: Doubtful.
Person B: Huh... this is an Ultimaybe. I'll never know what the right choice was, ever.