A reply you say when somebody asks you to do something that you do not want to do. Usally it is something that requires hard work or something that you do not want to do. You claim that if you do the favor, a one in a million event that causes harm will happan to you. The event will almost never happan but it is slightly possible. A more polite and more annoying way to say no.
Mike: Hey Josh, did you and George go on that roller coaster.
Josh: We were but George used the One in a Million Clause and claimed that he might fall off and die.