Harry Potter Half-Blood Prince fiasco
There's no way to know the correct answer. However, it would probably be a similar word to fiasco, only more negative and derogatory towards Snape. Perhaps "waste of time," "awful," or "catastrophe." But again, there's no way to know.
Considering that Harry was impolitely leaving out Snape's "professor"-title, he probably wanted to finish the sentence with a rude word or even a swear word.
The only way to know what Harry really wanted to say is to ask the author, I suppose.