How did 'sick' come to mean 'awesome' or 'really good / cool' in modern U.S. slang? I'm interested in origins and possibly regional patterns, if applicable.
This usage reminds me of the use of 'bad' to mean 'totally awesome' in the 80s. It would be interesting to know how that came about as well, and if the pattern is related...
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The OED says this slang is now especially used for skateboarding and surfing, and the first quotation is from a 1983 UNC-CH Campus Slang by the University of North Carolina, Chapel Hill:
Sick, unbelievably good: The Fleetwood Mac concert was sick.
Partridge notes bad is much older, and the OED gives the source as George Ade's story of a black shoeshine boy, Pink Marsh : a story of the streets and town (1897):
She sutny fix up a pohk chop 'at's bad to eat.
It says its originally US slang and means something good or excellent, especially stylish or attractive. The later quotations trace its use through black and jazz slang (1928, 1955, 1959, 1971 and 1989) until more 'mainstream' use is noted in a US newspaper in 1995 and a UK book in 2006.
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I think it was originally a skateboarding slang to express "shock and awe" after seeing something cool. I'm hazarding a guess that it was first used to describe crashes. The only corroboration I can find is from About.com and a Straight Dope forum post. Quoting the latter:
I first heard the word "sick" being used as slang on the 1987 skateboarding video put out by Powell and Peralta entitled "The Search for Animal Chin." A skater did some cool trick or whatever, and a hardcore skateboarder onlooker said, "that's sick". We thought it was hilarious, and I have heard it used ever since, though mainly among skaters/surfers/snowboarders/druggies and the like.
It is my observation that the word not so much means "cool", but carries a connotation more extreme than just that. It is used to describe something that is unbelievable, unprecedented, or just plain mind-blowing.
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I think the pattern is related, though I'm unable to substantiate that. Still, I've observed it enough: some adjective is used informally to mean something different than it typically means (maybe even the opposite of what it usually means) – a cool motorcycle, a nasty curveball, a rad(ical) dress, a wicked dance move, a gnarly book, an epic sunset, a sick jump, a bad pizza, etc. Somehow, the word sticks for awhile, probably because it sounds innovative. Yet as the popularity of the word swells, its freshness wanes, and it becomes ripe to be displaced by something new.
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This question ought to be reopened, because the current answers are basically wrong. Whether or not other usage in youth culture pre-dates it, sick became slang for pretty much the opposite of what it traditionally means in the late '90s in South London, with predominantly black kids into the 'grime' music scene, which in turn spawned the 'dubstep' music scene. Dubstep has since become popular in the USA, and the American kids that use this word tend to be into dubstep, which originally comes from South London. When I was a kid in the '80s, 'wicked' developed in very much the same way.