Meaning something is REALLY cool. And by playing with the double meaning of the word "sick" (along with "REALLY cool", the original meaning simply meant "unwell", etc) , emphasises how cool the thing in which is "sick" really is.
Sicker than the disease AIDS apparently.
That's what creates the "REALLY" presence in front of "cool".
Damian: "You should come check out the 30 pizzas I dumpstered from Little Caesers that I have in my car."