GEEWUN is a snarky, sometimes even derogatory, fan term for people who seem to have knee-jerk negative reactions to, and continuing difficulty in tolerating any of the post-Generation One (aka "G1") incarnations of the Transformers property.
It is not, as is sometimes believed, a catch-all nickname for anybody who likes Generation One, even though it was frequently used as such.
While the phenomenon is nothing new, when the 2007 live-action movie was in production, the fandom experienced an ongoing influx of new fans, who in their prepubescent ignorance, had no idea what older fans got upset over, or why characteristics that had always been in all previous incarnations had been thrown out the window.
These newbies considered fans who had stuck with the brand for 25 years to be mindlessly angry "Generation Y" members who seemed to regard "Transformers" not as a still-evolving cultural institution and business property, but as a sacred relic of their halcyon, Reagan-era childhoods.
bitter young new fanboys considered anyone who criticized the Bayformers movies as bitter people who made their voices heard by denouncing the movie because it would ruin Transformers forever. A common way of dismissing these types of comments would be to reference Cliffy B saying that Michael Bay "raped their childhoods" and "WAAH GEEWUN". The singularly unfortunate phrase "[so-and-so] RAPED MY CHILDHOOD!!!!!" even though not used by this group quickly became associated with them because of all the criticism by this group's detractors.
For some reason, many GEEWUNNERS were particularly incensed by the fact that Optimus Prime (the truck guy, not the tape guy) had flames painted on him, a detail that Michael Bay claims to have received death threats over (with that in mind, it really does make the fandom look amazingly sad). It probably would have been okay if the flames were burning down some forests, though.
Alas, the term is often trotted around to support the persecution complex of said people, who usually misconstrue the term to make it appear worse.