1. To be dead, destroyed, or in deep trouble after something negative happens. Almost always used in a figurative sense. I just got bad news from the auto repair shop—my car is history. You'll be history once the principal finds out you plagiarized that paper.
2. To be a thing of the past; to be no longer relevant. A: "I thought that you didn't get along with Jenny." B: "Oh, that's history! We're friends now. Can you please clean out all this junk? Cassette tapes are history, and there's no reason to keep them!
See also: history