c. 1300, "story, narration," from Old French prose (13c.) and directly from Latin prosa, short for prosa oratio "straightforward or direct speech" (without the ornaments of verse), from prosa, fem. of prosus, earlier prorsus "straightforward, direct," from Old Latin provorsus "(moving) straight ahead," from pro "forward" (from PIE root *per- (1) "forward") + vorsus "turned," past participle of vertere "to turn" (from PIE root *wer- (2) "to turn, bend").
The meaning "prose writing; non-poetry" (as opposed to verse or metric composition); "the ordinary written or spoken language of people" is from mid-14c.
"Good prose, to say nothing of the original thoughts it conveys, may be infinitely varied in modulation. It is only an extension of metres, an amplification of harmonies, of which even the best and most varied poetry admits but few." [Walter Savage Landor, "Imaginary Conversations"]
The sense of "dull or commonplace expression" is from 1680s, out of the earlier sense of "plain expression" (1560s). As an adjective, "relating to or consisting of prose," by 1711. Prose-writer is attested from 1610s; those who lament the want of a single-word English agent noun to correspond to poet might try prosaist (1776), proser (1620s), or Frenchified prosateur (1880), though the first two in their day also acquired in English the secondary sense "dull writer."