Bah, none of them cite the origins of the word.
It came from the old Transformers name calling due to having a similar "my nostalgia!" argument. It was during the Beast Wars era. G1 fanboys would troll Beast Wars fans saying "TRUKK NOT MUNKEY" to which the BW fans would simply ignore them and call them as "GeeWunners".
It still bugs me that someone felt the need to mutate the word during the transmission. "Geewun" rolls off the tongue far better and sounds a bit less like an aardvark is saying it.