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Refers to pop culture that has become stale. The "flat" part of the term is related to the state of soda once it has lost its fizz(and thus doesn't taste very good any more).
Once a bubble of pop culture has popped, it's flat culture.
"Did you hear about (something that's been in the news for a week)?"
"Man, you must be kidding. That's flat culture now!"
by Moologie August 23, 2007