A monepic sentence is not epic. Quite the contrary actually. It is a sentence formed of a single (monos in Greek) word (epos in Greek).
Baby talk is very monepic, with a lot of sentences such as “Mama!” or “Monkey!”
But surprisingly, a lot of sentences are monepic, particularly in conversations. For example, “What?”, “Yes!”, “Thanks.”, “Wow!” and “Epic!” are all monepic.
Finally, some people like to write “monepic poems”. I even found this poem formed of monepic and monosyllabic sentences, that still manages to tell a whole story. Enjoy!