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.
Human Biology student: My lab's at Sickkids. What about you? :)
Neruoscience student: WTF How did YOU get in there?!?! I'm at Krembil >:(
Human Biology student: Oh! Krembil is good though! My friend actually...
Neuroscience student: stfu your dad probably works at sickkids you nepo baby >:o
Human Biology student: No my dad's dead... :(