You pronounce it: [proh-zey-ik] IPA:/proʊˈzeɪ ɪk/
1. commonplace or dull; matter-of-fact or unimaginative:
a prosaic mind.
2. of or having the character or form of prose, the ordinary form of spoken or written language, rather than of poetry.
Origin: 1650-60; Late Latin prōsaicus.
I found it while reading “Where’d you go, Bernadette” by Maria Semple:
Fox’s preoccupation with the prosaic caught the attention of Professor Michael Graves, who hired her to work in his New York Office. (104)