Let’s say you bare your ass for the cosmos and let one rip, how much thrust will your gas produce?
If you’re only baring your ass through some sort of porthole with the rest of your body comfortable in a pressurised suit/capsule, the pressure difference between your body wall and the vacuum of space might prolapse your entire viscera out of your anus. This might not be the way you intended to gain thrust, but a sufficiently powerful prolapse (literally “letting one rip”) might act like a reaction engine and provide a significant amount of thrust. 
I’d wonder how much propulsion you’d get if you were inside the ISS and let one rip. I recall picking up from some of my readings (maybe from A Man On The Moon), the fuel cells in the Apollo service module, besides electrical power, produced water that had some extra H2 in it. Not enough to harm anyone drinking it, but enough to pass on through one’s system and exit. Though, that could have been from the food provided for the crews also. I believe I also read at some point there was a skirmish between the flight crews and the nutritionists who made the flight rations. It was resolved after some power higher up (maybe Slayton, maybe Chris Kraft) made the nutritionists live off the rations they made for 2 weeks. The flight crews won that one.
I found a similar (if not nearly identical) thread on the dope…
Farting is not much help apparently. Ejecting a liquid or throwing a solid with substantial mass might be a better option.