radical (adj.)

"reliance on direct experience and observation rather than on theory;" 1650s, originally in a medical sense, from empiric + -ism. The original medical sense was depreciative: "quackery; the pretension of an ignorant person to medical skill." The depreciative quality carried over later into the general sense of "reliance on direct observation rather than theory," especially an undue reliance on mere individual experience. In reference to a philosophical doctrine which regards experience as the only source of knowledge from 1796.

Were I obliged to give a short name to the attitude in question, I should call it that of radical empiricism, in spite of the fact that such brief nicknames are nowhere more misleading than in philosophy. I say 'empiricism' because it is contented to regard its most assured conclusions concerning matters of fact as hypotheses liable to modification in the course of future experience; and I say 'radical,' because it treats the doctrine of monism itself as an hypothesis, and, unlike so much of the half way empiricism that is current under the name of positivism or agnosticism or scientific naturalism, it does not dogmatically affirm monism as something with which all experience has got to square. The difference between monism and pluralism is perhaps the most pregnant of all the differences in philosophy. [William James, preface to "The Sentiment of Rationality" in "The Will to Believe and Other Essays in Popular Philosophy," 1897]

1918, "x-ray dose unit," a shortened form of radiation (q.v.). The meaning "unit of absorbed dose of ionizing radiation" is by 1954, an acronym from radiation absorbed dose. As shortened form of radical (n.), it is attested in political slang from 1820. Teen slang adjectival sense of "extraordinary, wonderful" is from late 1970s (see radical (adj.)).