What is the X in X-mas?
"steep slope," in northern England especially "the sides of a hill," early 14c., from Scottish, "slope, river bank," from Old Norse bra "eyelash," cognate with Old English bræw "eyelid," German Braue "eyebrow" (see brow). "The word must have passed through the sense of 'eye-brow' to 'brow of a hill', supercilium (cf. OE. eaghill 'eye-hill'=eyebrow)" [OED].