The sound one makes in response to something peculiar that has just occurred or has just been said. Most often used when irritated. Most often a natural response/reaction.
1) I thought I had a $5 bill in my wallet but it's not here .... hmpf.
2) Val: Just got an email from Sunny. Ted: hmpf
A sound uttered in disgust, sarcasm, irritation, or stubborness. "hmm" + "ffff" = "HMPF"
(invented by C. Erickson)
Sarah was irritated by the manner of her friends teasing, so she muttered an indifferent, "hmpf."