A fork of Rural Dictionary
When you are utterly disgusted and/or in disbelief you put your lips together and expel wind from your pursed lips making a quasi flatulence noise to demonstrate how you feel about what you just heard/ witnessed.
William: "So I totally banged this ten last night, Giselle quality, if not hotter, seriously" Michael (blowing wind): "phhhhhhhh! ya right, there's no way you could pull that off......."