X-man, n. — Green’s Dictionary of Slang

X-man n.

[such men are not ‘real’ men but ‘(e)x-men’; note also X-Men Marvel comic book series (f. 1963), made into a film in 2000, where the eponymous heroes are mutants]

(UK juv.) a male homosexual.

2001

OnLine Dict. of Playground Sl. 🌐 x-men n. homosexual.