Well according to the dictionary (yes, I checked all of them) it is: An inference or conclusion that does not follow from the premises or evidence. A statement that does not follow logically from what preceded it.
So - since this is flu season, and I have had my flu shot, I decided to try to write something using this mechanism and cold medicine. Um. Yeah, it was a writing prompt from a few years back, and sounded like a cool one.
Will post up what I came up with. Feel free to steal this idea, as this is the Internet. I stole it first.