Blow it(fail at something)
How did you do on the history exam?
I think I blew it! There was a section on the Civil War, and that's the chapter in the book that I studied the least.
Well, at this point you can't really be sure that you completely failed the examination. You must have done okay on the rest of the test.
Perhaps I didn't fail the entire exam, but I'm sure that I didn't do well.
entire = whole