1902, "model to display clothes," from French mannequin (15c.), from Dutch manneken "model of the human figure used by artists," literally "little man" (see manikin). A French form of the same word that yielded manikin, and sometimes mannequin was used in English in a sense "artificial man" (especially in translations of Hugo). Originally of persons, in a sense where we might use "model." Later (by 1931) of artificial human model figures to display clothing.
A mannequin is a good-looking, admirably formed young lady, whose mission is to dress herself in her employer's latest "creations," and to impart to them the grace which only perfect forms can give. Her grammar may be bad, and her temper worse, but she must have the chic the Parisienne possesses, no matter whether she hails from the aristocratic Faubourg St. Germain or from the Faubourg Montmartre. ["The Bystander," Aug. 15, 1906]
late 14c., "originating in the root or ground;" of body parts or fluids, "vital to life," from Latin radicalis "of or having roots," from Latin radix (genitive radicis) "root" (from PIE root *wrād- "branch, root"). The basic sense of the word in all meanings is "pertaining or relating to a root or roots," hence "thoroughgoing, extreme."
The figurative meaning "going to the origin, essential" is from 1650s. The political sense of "reformist" is by 1817, of the extreme section of the British Liberal party (radical reform had been a current phrase since 1786), via the notion of "change from the roots" (see radical (n.)). The meaning "unconventional" is from 1921. U.S. youth slang use is from 1983, from 1970s surfer slang meaning "at the limits of control."
The mathematical radical sign, placed before any quantity to denote that its root is to be extracted, is from 1680s; the sign itself is a modification of the letter -r-. Radical chic is attested from 1970; popularized, if not coined, by Tom Wolfe. Radical empiricism was coined 1897 by William James (see empiricism).