When you take a dump, then jack off in the bowl after you're done. Usually this is to save water and not flush twice, or to cover up for anyone in the house.
Can also be taking a dump right after you finish jacking off.
"I'm gonna go take a dunp-n-jack"
"Cool. See you in 30"
"I told my mom I was in the bathroom when I did my dump-n-jack"