What is the X in X-mas?
Old English balca "ridge, bank," from or influenced by Old Norse balkr "ridge of land," especially between two plowed furrows, both from Proto-Germanic *balkan-, *belkan- (cf. Old Saxon balko, Danish bjelke, Old Frisian balka, Old High German balcho, German Balken "beam, rafter"), from PIE *bhelg- "beam, plank" (cf. Latin fulcire "to prop up, support," fulcrum "bedpost;" Lithuanian balziena "cross-bar;" and possibly Greek phalanx "trunk, log, line of battle"). Modern senses are figurative, representing the balk as a hindrance or obstruction (see balk (v.)). Baseball sense is first attested 1845.
late 14c., "to leave an unplowed ridge when plowing," from balk (n.). Extended meaning "to omit, intentionally neglect" is mid-15c. Most modern senses are figurative, from the notion of a balk in the fields as a hindrance or obstruction: sense of "stop short" (as a horse confronted with an obstacle) is late 15c.; that of "to refuse" is 1580s. Related: Balked; balking.