On My Grandma!!
When you run out of things or people to swear upon. In other words, When you know you're lying but you already put it on God, Your Momma, Yourself, Your Soul, Your Dad, Your Gang and all you have left is your grandma. Poor Grandma, I bet you are scared of putting it on her, because you still live with her.
Example:
Bruh! Hey Bruh! I have 20 Flat Screen TV's and 3 Lamborghinis, we is Lottery world champions, on my Grandma!!
Bruh! Hey Bruh! I have 20 Flat Screen TV's and 3 Lamborghinis, we is Lottery world champions, on my Grandma!!