swear on (one's) mother's grave(redirected from swear on their mothers' graves)
swear on (one's) mother's grave
To make a very serious, solemn pledge, especially that one is telling the truth. Janet has sworn on her mother's grave that she wasn't the one to betray me, and I believe her. I swear on my mother's grave that if I have a chance to help your campaign, I will.