The gradual build-up of dry deposed fecal particles, released through flatulence, these particles act in accordance with the random behavior detailed by Brownian motion (modeled by the 1D Smoluchowski model) to the extent that they eventually form a visible, tangible lump of feces.
Pre-requisite: An extremely large quantity (calculated to be approximately 10^12000) of reoccurring flatulence within the same locale.
Occurrence?: So far has not been witnessed in nature or a controlled setting, but is theoretically possible.
Alternative definition: When you fart you release "poo particles" and if you fart onto the same spot enough times they build up and make a full turd.