Hammer a nail into my chin, if it ever happened. Informally they say, Spit on my grave if it ever happened.
Someone who is so confident that his following statement is irrefutable and cannot be gainsaid. He would precede by saying it. It's to persuade and convince. I'm looking for a formal way of saying this in English.
Hammering a nail into one's chin is in some parts of the Arab World goes back to a pre-Islamic custom that shames people.