Bet on the Gambler
When someone with intestinal issues (e.g. diarrhea)attempts to fart and up until the last instant doesn't know whether it will be a shart or a fart, in effect gambling on the outcome. Different than a shart, however, the results of losing the Bet on the Gambler can be significantly more catostriphic. 
The large stain in Lucy's pants made it clear she Bet on the Gambler and lost. 