1650s, "having to do with prose" (a sense now obsolete), from French prosaique (15c.) and directly from Medieval Latin prosaicus "in prose" (16c.), from Latin prosa "prose" (see prose). Meaning "having the character of prose (in contrast to the feeling of poetry)" is by 1746; the extended sense of "ordinary, commonplace in style or expression, lacking poetic imagination or beauty" is by 1813. Both sense are from French. Related: Prosaical; prosaically.