Swear on (one's) mother's grave verb phrase
If someone swears on their mother’s grave, they pledge that they are telling the truth.
He swore on his mother’s grave that he never stole anything!
I swear on my mother’s grave that I have never seen her before.
If you give me another chance, I swear on my mother’s grave that I will not make this mistake again.
Godfrey Daniel means God Damnit or God Damn it.
Used to say that what you're saying is true, or that you'll do what you promise .
To do what you promised to do or what people expect you to do
Offensive, or profane language
The verb "swear" should be conjugated according to its tense.
The idea of the expression may come from the fact that people tend to have more respect for the dead, especially for their mother. So, when they swear on their mother’s grave, they are completely honest. If not, it would be totally disrespectful to use their mother’s name for a lie.