1. From being ridiculous, irrational, incongruous, or illogical: When people argue ab absurdo, they are trying to establish the validity of their positions by pointing out the ridiculousness or foolishness of their opponents' arguments.

Although an argument ab absurdo "demolishes" an opponent's position in debate, it does not necessarily prove the validity of that person’s position. The only thing that may be accomplished is that the ab absurdo argument may make the other person seem to be foolish or illogical.

2. Etymology: Latin "from the absurd" or "from nonsense".