Green’s Dictionary of Slang

radical adj.

[SE use of radical as ‘basic, essential, from the roots’ and eschewing political overtones; like a number of other terms, the word moved from surfer jargon, to ‘Valley Girls’ use and thence, via the 1990s Teenage Mutant Ninja Turtles craze, to general use]

(US campus) a term of utmost approval.

[US]Eble Campus Sl. Mar. 6: radical – surpassing the usual image of modern and new [...] This is a radical album.
[US]‘Jennifer Blowdryer’ Modern English 34: radical (n): A word that college boys use, not radical like Jerry Rubin [...] radical more like an especially colorful Hawaiian shirt.
[US]Lerner et al. Dict. of Today’s Words.
[UK]G. Iles Turning Angel 130: If you’re going to mess with him at all, you want somebody like Dr. Elliott around. Somebody who can get radical if things get out of hand.