What is the X in X-mas?
early 15c., from Middle French fraternel and directly from Medieval Latin fraternalis, from Latin fraternus "brotherly" (see fraternity). The noun meaning "fraternal twin" is recorded by 1911.
fraternal fra·ter·nal (frə-tûr'nəl)adj.
Of or relating to brothers.
Of, relating to, or being a twin developed from two separately fertilized ova; dizygotic.