Several definitions given here are correct in connecting the term to radical but incorrect in the date of origin. Being old enough, I can attest that it was used at least as far back as the seventies.

:/

The third person in a relationship

It is rare to use. It's a cool slang word from the 1970s/80s. It means "awesome", "exceptional", "cool". It's something American surfers or skateboarders would say.

1: Something that should be left in the 90s

2: Something to avoid in Fallout

A totally rad 80's movie about BMX racing featuring that hot chick from Full House

A more excting way to say 'cool' or 'awesome' mostly used back in the 80's. On the show Full house.