When someone shoves one of their fingers up their arsehole, then pulls it out to see if shit gets on their finger or not in order to decide something one way or another. A game of chance, of sorts, like flipping a quarter to decide or determine something. If shit gets on your finger the answer is "No", but if you pull it out clean the answer is "Yes".
Well, I don't know if I should go to the show tonight or not. I guess I'll have to decide using the Fecal Finger of Fate.