When two people are out together, and one has romantic intentions but isn't sure that the other person does as well. The one with romantic intentions is afraid to ask the other person if he/she also has romantic intentions, because the other person might clarify that he/she DOESN'T. And that would lead to an awkward situation.
The word is a combination of Heisenberg, the scientist famous for his Uncertainty Principle, and the word date.
I thought I was getting somewhere with Jennifer last night, but it turns out it was just a Heisendate.