The word origin is from the west cost of Saudi Arabia ( Jeddah) then moved to the north to Syria, Lebanon , Palestine (where apparently they use it the wrong way).. etc ..
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 : Give me your car keys.
Ali : Kus Emek!! I dont want your fuckin car.
A curse word, used in Hebrew as well as Arabic, meaning Your mothers pussy.
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, i have a test on monday"
"kus Emek,Ya Ben Zona"- An elaboration