1) The condition of speaking the truth and having no one believe you.
2) The condition of being able to predict the future, be it the outcome of a particular event, or the reactions of others to the same event, and having no one believe your prophecy until it transpires.
3) Being able to see or understand things long before others, often resulting in them coming to the same conclusions long after your own initial analysis.
(All definitions come from Cassandra, the queen in Greek mythology who was appointed by Apollo with an inability to lie, yet cursed by having no one believe her prophecies.)
"I told him she was going to dump him if he didn't shape up, but no one ever believes my Cassandra syndrome."