kisumuk

Actually its not the most offensive swear word in arabic.

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.

This word is one of the most offensive 'swearwords' in arabic. It is very disrespectful when used, as it usually starts a fight. It basically means 'your mothers vagina' but in a much vulgur way.
But between friends, it is used in an innocent way.