'rad' originates from the word 'radical', which in the 1980s was shortened to 'rad' in American slang, where 'radical' meant 'excellent' or 'impressive'.

'radical' was used in the 1970s and 1980s to describe something extreme or excellent, and it was eventually shortened to 'rad' in colloquial speech.

Initially, 'radical' meant 'extreme' or 'fundamental', but over time it evolved into its current slang meaning of 'excellent' or 'impressive'.