More often than not it is a quick way to provoke someone and a sure way to cause a fight.
Sometimes shortened to just "your ma se...", which is just as effective. Or even expanded as in "jou dubbele ma se poes".....
Also heard often when someone hurts hurts themself, e.g stubbing your toe....."Eina!! Ma se poes!! That hurts!!
Dirk: What did you say about my mother? I'm gonna fuck you up!!!
Spectator: This ma se poes guy wants to fight. I'm sick of these ma se poes nights out!!