phrasal verb

sick

impressive, especially because of being fashionable or attractive

If you're wearing a new outfit and someone under twenty-five describes it as sick, then there's no need to rush out of the room in despair. Quite the opposite in fact, because the comment suggests that you've definitely hit the spot in your choice of clothing and can be confident that you're 'looking good' in the eyes of the beholder.

language data suggests sick is also common in the context of sports popular with young people, such as skateboarding and snowboarding

A trendy new synonym for adjectives such as excellent, awesome and cool, sick now has a new sense which forms the direct antithesis of its established meaning of 'unpleasant' or 'upsetting' (e.g. a sick joke). This positive sense of sick pops up in speech and in informal written contexts (online discourse etc) and is used mainly, though not exclusively, by young people. Common collocates are clothing and fashion items, or the latest electronic gadgetry: mobile phones, computers, MP3 players, etc. According to research undertaken by Oxford University Press, language data suggests it's also common in the context of sports popular with young people, such as skateboarding and snowboarding.

Unlike sick in its conventional meaning of 'unpleasant', the positive sick refers to an absolute quality and so doesn't usually occur in the comparative or with grading adverbs like very, slightly, extremely, etc. This means for instance that although it's possible to describe something as a very sick joke, you'd be highly unlikely to hear someone talking about a very sick outfit.

Background – sick

The positive use of sick first appeared in the US in the early eighties, but didn't emerge in British usage until relatively recently. It follows in the wake of wicked and bad, two other adjectives which have taken on a popular 'inverted' meaning (i.e. a meaning which represents the direct opposite of one of their other meanings). Such inverted meanings turn out to be older than you might expect – the positive use of wicked, for instance, dates as far back as 1920, when it was first used to mean 'wonderful' by author F. Scott Fitzgerald in his first novel This Side of Paradise.

The positive use of adjectives which started life as negative is one realization of a concept which linguists technically refer to as auto-antonymy. Antonyms are words with opposite meanings, and auto-antonyms are therefore 'words which are the opposite of themselves'. Auto-antonyms are sometimes also referred to as contranyms (also spelt contronyms) or Janus words (from the name of the Roman God with two heads facing in opposite directions). Auto-antonyms occur in various parts of speech and are more common than you might realize – an example in everyday use is the adverb quite, which can mean 'slightly' (e.g. quite nice) or 'completely' (e.g. quite fantastic), or consider the verb clip, which can mean both 'fasten together' (e.g. Clip the microphone to your shirt.) and 'cut off' (e.g. I need to clip my nails.).

The concept of auto-antonymy is one sub-type of a wider phenomenon described in linguistics as semantic change, which refers to the evolution of a word's usage in terms of changes in meaning. Other types of semantic change include narrowing, where a word takes on a meaning which is more specific than its original use (e.g. in Old English, the word meat referred to food generally, but now refers to a particular kind of food), and widening, where a word for a specific item becomes used as a general reference (this is often seen with brand names, such as hoover as a general term for using a vacuum cleaner, or google to describe the activity of using an Internet search engine).