What is the X in X-mas?
1550s, "line of battle in close ranks," from Latin phalanx "compact body of heavily armed men in battle array," or directly from Greek phalanx (genitive phalangos) "line of battle, battle array," also "finger or toe bone," originally "round piece of wood, trunk, log," of unknown origin. Perhaps from PIE root *bhelg- "plank, beam" (cf. Old English balca "balk;" see balk (n.)). The Macedonian phalanx consisted of 50 close files of 16 men each. In anatomy, originally the whole row of finger joints, which fit together like infantry in close order. Figurative sense of "number of persons banded together in a common cause" is attested from 1600 (cf. Spanish Falangist, member of a fascist organization founded in 1933).
phalanx pha·lanx (fā'lāngks', fāl'āngks')
n. pl. pha·lanx·es or pha·lan·ges (fə-lān'jēz, fā-)
Any of the long bones of the fingers or toes, numbering 14 for each hand or foot: two for the thumb or big toe, and three each for the other four digits.