It can basically mean both good or bad depending on the way you use it. it's like saying gay if it's "bad" without using that word. Or if your friend has a new game or hot shirt from a band with it's signature can be called "sicktits".
It was basically made when someone slurred the words "sicko" and "attitude" together.
Dude that fucking shirt is so Sicktits!
WTF that's Sicktits!