What is the courtcase that set the precedent for Affirmative Defense?

Answer 1

Don't hire Mr Laughter for your attorney... (hehehe)

Directly from the Law Library:
"An assertion in the defendant's answer to a complaint that the plaintiff's suit is barred, even assuming the alleged injury or loss, on certain legal grounds. These grounds include the defendant's duress or necessity, the plaintiff's assumption of risk, the plaintiff's contributory negligence, the expiration of a statute of limitations, or other legal justification of the defendant's conduct. Affirmative defenses must be separately identified and asserted in the defendant's answer".

This aspect of law has evolved, over many court cases, as legislation, as opposed to case law.