Slang for 'radical,' meaning excellent, cool, or impressive; also a unit of radiation absorption.
As slang, shortened from 'radical' in the 1980s. As a scientific term, 'rad' is an acronym for 'radiation absorbed dose,' created in the mid-20th century.
The slang 'rad' peaked in the 1980s skateboarding and surfing culture, but the scientific 'rad' is still used by physicists—it's one of those rare words where slang and technical jargon developed completely independently for different communities.
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