The X-Men comic book series was created by writer Stan Lee and artist Jack Kirby, and first appeared in September 1963. The X-Men were a group of mutant superheroes who were born with extraordinary abilities, and were often hated and feared by non-mutant humans. The X-Men were led by Professor X, a powerful telepath who sought to teach mutants to use their abilities for the betterment of humanity. The series tackled themes of prejudice, diversity, and acceptance, and became one of the most popular and enduring comic book franchises of all time.
The extra power they possess is the classic explanation for the X-Men appellation, which Professor Charles Xavier first used in X-Men #1 by the renowned team of Stan Lee and Jack Kirby (even if most people in-universe and out likely assume they were named for the X-Gene, or for Xavier himself). This was probably Stan's hasty justification for the eye-catching moniker, but it hasn't been brought up much since. With Xavier's changing persona and the novels' more somber tone, it has also seemed increasingly out of character for the squad to be called after some simple pun. Is the source for the names of Marvel's merry mutant members one of the many Charles Xavier concealing dark secrets plans over the past two decades?
An alternative was provided in Immortal X-Men #8, written by Kieron Gillen and Michele Bandini. The issue, which is set in late 19th-century London, covers Mystique and Destiny's tumultuous but long-lasting relationship and has their first encounter with Nathaniel Essex, who would later become Mister Sinister (and is in the process of doing so in the story). When the two learn that Essex's growing shadow self is responsible for a run of deaths, Mystique confronts him while Destiny impersonates Irene Adler. Mystique then uses her powers in front of him. "You're... Essex-Men," the startled Victorian scientist stammers, to which Irene responds in the affirmative.
In too many narratives to list, Nathaniel Essex's relationship with Charles Xavier, Scott Summers, and Jean Grey has been well-documented. Essex was a well-known and esteemed scientist before he became Mister Sinister, and the two of them corresponded about evolution and natural selection. According to Essex's history, he was the first scientist to formulate a theory regarding mutants as a developing subspecies of humans. He was derided for his theories, and mutants were mostly unknown until the modern era. It's extremely clear that Charles Xavier was exposed to Essex's well-known previous work on the so-called Essex Factor," given that he was a pioneer in the modern study of mutants (and encountered similar resistance to his beliefs while a student). The term "Essex-Men" was probably known to his scientific contemporaries given that Irene and Raven don't seem perplexed by the change of phrase.
The issue provides a compelling justification for an alternative history of the X-Men moniker, even though it has not yet been confirmed on the page: When Xavier came across Essex's writings, he became motivated. However, given that the individual has now gained a very bad reputation, Charles chose to shorten the name in order to avoid tarnishing his own research on mutant evolution by conjuring a discredited scientist. Thus, "Essex-Men" became "X-Men," and "Essex Factor" became "X-Gene." As far as secrets go, this isn't among Charles' top 20, and it makes sense that he didn't want to link Essex to his own work or students. It would be incredibly ironic if Nathaniel had anything to do with their names, given that he ended up turning into one of the X-fiercest Men's foes. Among his scientifically inclined contemporaries, his idea was probably known by the "n" nickname.
Immortal X-Men from Marvel Comics is available now