Popularized as a joke from SMBC Comics; comes from an amalgam of the words "dumb" and "epiphany."
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?
And so on.
"Possibly, but... wait, what? You... Ohhhhhhhhh, I just had a Dumbpiphany"
Credit to Zach Weiner of the Saturday Morning Breakfast Cereal webcomic (SMBC).
Alex: Possibly, but...wait, what? You...OHHHHHHHHH!
(Alex has had a dumbpiphany.)
-Possibly, but...wait, what? You....OHHHHHHHHH. Just had a total dumbpiphany.
-Walter had a dumbpiphany that discussing flying elephant theory with Jim was fucking retarded.