What is the X in X-mas?
late 14c., "upright, not bending," from Latin erectus "upright, elevated, lofty; eager, alert, aroused," past participle of erigere "raise or set up," from e- "up" + regere "to direct, keep straight, guide" (see regal).
c.1400, a back-formation from erect (adj.) or else from Latin erectus. Related: Erected; erecting.
erect e·rect (ĭ-rěkt')
Being in or having a vertical, upright position.
Being in or having a stiff, rigid physiological condition.