The superlative form of sick; either most ill, or slang for the most impressive or cool.
From Old English 'seoc' meaning ill or diseased. Modern slang use (meaning 'awesome') emerged in hip-hop culture in the 1980s as a playful reversal of meaning.
Calling something 'sick' as a compliment flips the word's meaning completely—linguists call this 'semantic inversion,' and it happens often in youth slang as a way to create in-group identity.
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