An Exclamatiion, normally good in nature, refering to the virtue of a particular instance or action in accordance to its difficulty, risk or originality. Can also refer to an object that is the result of a "wicked sick" action (this particular usage does not necessarily denote a positive result). Similar in nature but not necessarily in definition to "w00t", as it is almost completely defined by the speaker/writer.
"30 kills and no deaths? Wicked sick!"
"A 1080 with an indy? Wicked Sick"
"Yeah he was fine but the car and the pillar looked wicked sick."
"His arm was all torn up" - "Wicked sick"