Discussion Forum—A Way with Words, a fun radio show and podcast about language

I can't answer your question about when this originated. I can tell you that it's not just the youth in London using the word "sick" this way. My friends and I, throughout junior high and high school, also used it this way. I grew up in Northern California and most of my friends were skateboarders. I remember hearing it used that way in the Bay Area for the first time around 1995.