Superlative form of 'chill,' meaning the coldest, or in slang, the most relaxed and laid-back.
From 'chill' plus the superlative suffix '-est.' The slang meaning of 'relaxed' developed in youth culture, particularly hip-hop, in the late 20th century.
The slang use of 'chill' is fascinating—'chilling' originally meant literally being cold, but now it means being super relaxed! Words have parallel lives where old meanings and new slang meanings exist at the same time.
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