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 Ninja Turtles never invented a single term. To suggest so is not rad but very uncool."
When you text your crush and tell them you like them and they say they don't like you with a five paragraph text. All you have in mind is rad.
You: Hey I like you do you want to go out?
Crush:Sorry I don't like you but could we still be friends? I don't want to hurt your feelings .
You:....Rad.......