What is the X in X-mas?
"fur or pelt of the European sable" (Martes zibellina), early 15c., from Middle French sable (also martre sable "sable martin"), in reference to the mammal or its fur, borrowed in Old French from Germanic (cf. Middle Dutch sabel, Middle Low German sabel, Middle High German zobel), ultimately from a Slavic source (cf. Russian, Czech sobol, Polish soból, the name of the animal), "which itself is borrowed from an East-Asiatic language" [Klein], but Russian sources (e.g. Vasmer) find none of the proposed candidates satisfactory.
"black" as a heraldic color, early 14c., commonly identified with sable (n.1), but the animal's fur is brown and this may be a different word of unknown origin; or it might reflect a medieval custom (unattested) of dyeing sable fur black. As an adjective from late 14c. Emblematic of mourning or grief from c.1600; c.1800 as "black" with reference to Africans and their descendants, often with mock dignity.