The full explanation is to curse the moment that someone came out of their mother, the fact that they were even born. Also can be used as a reply
"kus Emek,Ya Ben Zona"- An elaboration
Literally it means "your mothers vagina" , but its short for " shove it up your mothers vagina " its used to be " fee Kuss ummak "
Usage: said when someone refuses to give you something , you tell him , kuss ummak I don’t want it . as in you can keep it.
Ali : Kus Emek!! I dont want your fuckin car.