Has a small top and smell like rotten egg.
Look at this shroom, what a tinytop LOLAWLLWLL
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."