It's like a spoken idiom which means " I won't do that " or I will never do that " that's because peaches appear in Egypt only once a year and It's not abundant all over the year so If you are telling someone this it means when the peaches appear I will do it while you won't " the other person knows this too " 1)ممكن تنظفي غرفتك ؟؟ في المشمش can you clean your room ?? Impossible 2)متي ستذهبين معي ؟ في المشمش

Dina_mrs answered the Question perfectly :) in other words it also means "No Way"

it means "never going to happen" or "in you dreams