UC is an abbreviation for Universal Century; a fictional timeline used by Mobile Suit Gundam
and it's sequels, one of many series in a long running Japanese anime franchise.
UC fanboy refers to a fanboy
who believes that the original series and its subsequent sequels are the best of any series' in the franchise.
In Internet discussion boards, UC fanboys often regarding viewers who prefer non UC Gundam series' in a condescending manner, usually remarking that they have no taste or have no idea that the newer series' are all cheap knockoffs of the originals. Consequently, the biggest pet-peeve of a UC fanboy is someone who has never seen any of the UC series creating a topic about how they enjoyed the non-UC series in question
fun fact: someone claiming that Gundam SEED Destiny is the best series in a UC topic is often considered a troll