mmhhmmm is not much of a word but more of a peculiar sound most commonly made by women of African origin.
it is mostly used as a filler for the words yeah right and many other words. However it is usually used in a sarcastic or negitive tone typically towards another individual.
person 1: "hey baby sorry i was late traffic was bad."
person 2: "mmhhmmm i bet you was hanging out with that slutt Kesha we are through"