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The little piece of poo that is pushed out then broken off during a forceful fart. Similar to poop-nugget and shart. (It is the less wet and messy version of the shart)
"Oh man, I think I just left a fart rock in my jockey's."

"Dude, I can totally smell that fart rock you just dropped. Go to the bathroom and clean that out."
by Heather Dizzle January 21, 2008
A fart rock is actually a rock containing sulfur that you can break, or scrape, to achieve a horrible fart smell.
Someone broke a fart rock in the hallway and it was so bad that they had to close it off for the day.
by Diesel AG April 04, 2009