It was a glorious day when he bought a new bag of beef jerky. He's heart sank when the first tasty meaty morsel slipped out of his hand and landed in the dirt. He did not want dirty jerky.
A dirty Jerk occurs when you are taking a shit and decide "Yo I have to Jerk right now" and proceed to jerk off while still sitting on the toilet (preferably without flushing or wiping)
"Yo I was just in the mood and had to hit The dirty Jerk, I was just feeling it and something came upon me that could not be controlled"