(AOT/SNK reference) (Attack On Titan Season 4 spoiler) The sound that you produce when your childhood sweetheart calls you a “slave” and tells you how much he despises your existence.
Eren: (...) I couldn’t stand to look at an undoubting slave who only ever followed orders. Ever since I was a kid, Mikasa, I’ve always hated you.
A noise, pronounced as a kissing sound, used to confuse, startle, or otherwise fluster an enemy. The "mmfphvtt" sound is often accompanied by a fake ball tap in order to enhance the effect and further annoy the target.
Guy 1: Hey can you help me with this math problem?