When you are slacking off, it usually means you are being lazy and not doing what you are supposed to be. ex: I slacked off in class today and missed what the teacher said about the test, because I was too busy passing notes back and forth to my friend. definition: 1. Moving slowly; sluggish: a slack pace. 2. Lacking in activity; not busy No slacking: Keep busy, pay attention No slacking off!: You better pay attention and keep busy or else. I hope this helps you Nozturk! The above answer is correct as well :)

It's basically the same. "Slacking" means to be sloppy (or "not to be working carefully enough") with something. "No slacking" means "Do your job well!" "Slacking off" means to start being sloppy. (Or to stop being careful with your work). "No slacking off!" means "Don't stop doing your job well!" You can say these in situations where somebody is working for you, for example cleaning your ship: "No slacking! If it isn't 100% clean, you will do it again!"