c. 1300, "story, narration," from Old French prose (13c.), from Latin 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").
"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"]
Meaning "prose writing; non-poetry" is from mid-14c. The sense of "dull or commonplace expression" is from 1680s, out of earlier sense "plain expression" (1560s). Those who lament the want of an 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."