Mainly used in the Egyptian dialect of Arabic, this phrase means "no way!"
So if you visit Cairo one day, and want to try to haggle with one of the merchants in Khan-al-Khalili, you can keep saying "mish mumkin" when they offer you a price until they get to the lowest price.
Dude 1: "Man, yesterday I saw a Lamborghini Countach that was painted with a really glossy and shiny blackish-purple color, and it had spinning rims, and to top the back spoiler was shaped differently, and it looked really scary. Plus a huge flame came out of the exhaust once that sucker took off."
Dude 2: "Mish mumkin!"
Dude 1: "How could I lie about this man?"