Arguendo is a legal term that means "for the sake of argument." It is used within court cases, mainly within the appeals system, to show that something would still not be true due to something else.
An example of this is when one side shows for the sake of argument that even if the defendant did what the plaintiff says (such as negligence), that due to the actions of the plaintiff being even more negligent, the defendant by default is not the party at fault.
Arguendo is information that is covered within a court case or appeal that does not bear a direct effect on the main subject, but that is used to show an example.