The realization that the reason the entire conversation has been difficult to follow is that you're talking to an idiot, who is further claiming and/or asserting something which is blatantly unfounded or unintelligent; whereas before, you thought it was due to lack of understanding on your part.
Popularized as a joke from SMBC Comics; comes from an amalgam of the words "dumb" and "epiphany."
A "dumbpiphany" in action:
Jack: The reason my relationship with Sarah didn't work was obvious...she's a Gemini and I'm a Leo!
Frank: Well, yea- wait, what?! What about how you cheated on her constantly until she found out?