Stands for 3D: Difficult, Dangerous, Dirty.. Sometimes Demeaning or Demanding can replace one of the D's. I believe this came from Japan, but Koreans refer to some diffcult jobs or tasks as "3D".
"My job is so 3D, but I have nowhere else to go..."
"I realize what I'm asking you to do is 3D, but it'll be worth it at the end."
"Your face is 3D!"
