noun phrase. An attempted smile that’s not restaurant-quality.
Real citation: “If I don't know you and I pass you on the street, chances are, I try to flash you a friendly smile. The thing is, when I do this, I seem to have some problem coordinating my mouth muscles, and so what you get can best be described as an "mmmf" face, the sort of face that Charlie Brown makes when he is eating a peanut butter and jelly sandwich and grunting in assent.”
(June 2, 2008, Blogging While Feminist/Plain(s) Feminist, http://plainsfeminist.blogspot.com/2008/06/working-on-my-smile.html)
Made-up citation: “Researchers studying the mmmf face, the oooooh face, the grrrr face, the aaaiiiiieeeeeee face, and the moooooo face have determined that diverse expressions are correlated with various faces, though further research is needed.”