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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
by Lamop January 29, 2005
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."
by random-runner April 16, 2009