Prosaic (or the fancier version prosaical) means “of or having the character or form of prose rather than poetry” or “commonplace or dull; matter-of-fact or unimaginative”.
I hate how this word makes such a huge generalization. Poetry can be dull and unimaginative, too!
Well, according to the Online Etymology Dictionary, prosaic originally just meant “having to do with prose”. Then in 1746 it gained the meaning “having the character of prose (in contrast to the feeling of poetry)”, and in 1813 it gained the even-more-generalized meaning of “ordinary”.
But it sure isn’t an ordinary word. I think it’s best suited to literary topics, like prosaic poetry and poetic prose. Take a look at this example: “Prosaic kitchen chairs and stools can be transformed with shiny seat cushions”. If I looked at a plain chair, prosaic definitely wouldn’t come to mind; it just sounds forced in that context. Maybe that’s just me, though. What do you think?