What is the X in X-mas?
1580s, from Latin deficientem (nominative deficiens), present participle of deficere "to desert, revolt, fail," from de- "down, away" (see de-) + facere "to do, perform" (see factitious).
deficient de·fi·cient (dĭ-fĭsh'ənt)adj.
Lacking an essential quality or element.
Inadequate in amount or degree; insufficient.