It describes a situation where someone purposefully does something to complicate the life of someone else.
It also describes another way of saying how disgusting something or someone is. explaining how you feel.
Scotty shank nugs has also been used to mean hurling everything out of your stomach and then some... meaning that burning fluid called bile. Something a lot of people really don't like doing in whatever circumstance.
"I feel like scotty shank nugs"
"I was driving my car down Alafaya Trail when I had to stop in the middle of the road to ralf all over my front seat. That was a total scotty shank nug move."