Literally – this can be defined as charactersitics of prose as distinguised from poetry. So, the meaning can be extended to unimaginative / dull / everyday or ordinary kind of narratives of situations.
Way back in 1600s, any text that was not poetic was prosaic. Back then, “prosaic” was not having any aegative connotations and it simply indicated that a written work was made up of prose.
Later, poetry is viewed as the more beautiful, imaginative, and emotional type of writing, and prose was gained the status of mundane and plain-Jane (Terms used for unattractive women).
As a result, English speakers started using “prosaic” to refer to anything considered matter-of-fact or ordinary, and they gradually transformed it into a synonym for “colorless,” “drab,” “lifeless,” “lackluster” and “humdrum” (all convey similar meaning)
Example
Our prosaic lives have made us forget how to appreciate art
They broke up because their relationship was getting quite prosaic
He has defaulted on his rent payments so many times that even the landlord is now familiar with his standard and prosaic excuse.