GEEWUN is a snarky fan term for people who seem to have knee-jerk negative reactions to, and continuing difficulty in tolerating any of the post-Generation 1 (aka "G1") incarnations of the Transformers property.
It is not, as is sometimes believed, a catch-all nickname for anybody who likes Generation 1, even when it is used as a catch-all nickname for anybody who likes Generation 1.
While the phenomenon is nothing new, when the 2007 live-action movie was in production, the fandom experienced an ongoing influx of 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. These bitter young people made their voices heard by preemptively denouncing the movie...not because it would ruin Transformers forever, but because it would "rape their childhoods" by deviating from their rose-colored—and often incorrect—memories of the first two seasons of the original cartoon. The singularly unfortunate phrase "[so-and-so] RAPED MY CHILDHOOD!!!!!" quickly became one of the more memorable used by this group, and used in criticism by this group's detractors, in which it seems to have taken on a life of its own and overtaken the original usage...so it goes.
For some reason, many GEEWUNNERS were particularly incensed by the fact that Optimus Prime had flames painted on him, a detail that Michael Bay claims to have received death threats over. 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.
See also