They're used to mean, "great, amazing". If you want to describe something as great, you could call it "fully sick". This slang is even new to me. I think the idea is that it's so awesome and powerful it could make you sick. Or it is potent... like "dope" (marijuana). "Sick" has become "ill" some years back: "that band is so ill" = "that band is so amazing". I wonder if it will die out soon? Most "cool" slang does that in a matter of years.