The next level in the progression of the "your mother" insults, after "your mother's mother" or "your mother's face". Should be used in the same meaningless fashion as "your mother", perhaps when you've run out of insults, or need a step up from the lame "your mother" insult already directed at you.
Person 1: What up
Person 2: Your mother
Person 1: Yeah well your mother's mother
Person 2: OHHH well your mother's mother's FACE!
Person 3: BURN!! He's got you there...