c. 1300, fasoun, "physical make-up or composition; form, shape; appearance," from Old French façon, fachon, fazon "face, appearance; construction, pattern, design; thing done; beauty; manner, characteristic feature" (12c.), from Latin factionem (nominative factio) "a making or doing, a preparing," also "group of people acting together," from facere "to make" (from PIE root *dhe- "to set, put").
Especially "style, manner" of make, dress, or embellishment (late 14c.); hence "prevailing custom; mode of dress and adornment prevailing in a place and time" (late 15c.). Meaning "good style, conformity to fashionable society's tastes" is from 1630s.
To call a fashion wearable is the kiss of death. No new fashion worth its salt is wearable. [Eugenia Sheppard, "New York Herald Tribune," Jan. 13, 1960]
In Middle English also spelled faschyoun, facune, faction, etc. Fashion plate (1851) originally was "full-page picture in a popular magazine showing the prevailing or latest style of dress," in reference to the typographic plate from which it was printed. Transferred sense of "well-dressed person" had emerged by 1920s. After a fashion "to a certain extent" is from 1530s. Shakespeare (c. 1600) has both in fashion and out of fashion.