As ridethetalk demonstrates,
radness is listed by the online Oxford. It's also in Wiktionary. Its use is somewhat niche, but I'll accept it as a rare word.
The relevant sense of
rad originated in the 1970s. It was first used in skateboarding circles to mean challenging, risky, extreme - derived presumably from
radical. Over time its meaning was generalised to encompass anything admirable or cool. The derived term
radness has been around since the 1980s.
A recent example is from the music blog Stereogum in March this year:
They play classic rock and roll rippers with garage-rock swagger and raucous punk-rock savagery, and their new song “Tell The Papers” is three and a half minutes of pure, uncut radness. The indelible hooks and the bashed-out intensity complement each other perfectly, and you can practically smell the spilled beer drying on the floor.
