A prophet whose predictions are so wildly, obviously wrong that he is listened to for entertainment value - and doesn't realize.
A phrase I created a number of years ago, referring actually to Nostradamus. Because if you actually read his prophecies, they are about as meaningful as predicting a ham sandwich could have mayo.
...But now listed here, because I just saw someone else use the phrase in a comment at the brilliant Sadly No. And I figured I should at least put my stake in the ground here. As much as it's weakened by the honesty of admitting at least one other human came up with it too.
I couldn't have predicted that. I'm no Nostradumbass.