rad·i·cal Pronunciation: ˈra-di-kəl Function:adjective Etymology:Middle English, from Late Latin radicalis, from Latin radic-, radix root — more at rootDate:14th century
1: of, relating to, or proceeding from a root: as
a: (1): of or growing from the root of a plant <radical tubers> [or] (2): growing from the base of a stem, from a rootlike stem, or from a stem that does not rise above the ground <radical leaves>
b: of, relating to, or constituting a linguistic root
c: of or relating to a mathematical root
d: designed to remove the root of a disease or all diseased and potentially diseased tissue <radical surgery> <radical mastectomy>
2: of or relating to the origin : fundamental
3 a: marked by a considerable departure from the usual or traditional : extreme
b: tending or disposed to make extreme changes in existing views, habits, conditions, or institutions
c: of, relating to, or constituting a political group associated with views, practices, and policies of extreme change
d: advocating extreme measures to retain or restore a political state of affairs <the radical right>
4slang : excellent, cool
— rad·i·cal·ness noun

Like so many words, this one has apparently changed in usage due to misunderstanding. You'll notice that 3a is the polar opposite from 3d. My take is that radical and 'fundamentalist" mean one in the same thing. Going back to one's roots or fundamentals means a return to the foundation or underpinnings, e.g., the Constitution, governing documents, essential tenets. -JC