What is the X in X-mas?
1782, from French déficit (late 17c.), from Latin deficit "it is wanting," an introductory word in clauses of inventory, third person singular present indicative of deficere "to be deficient" (see deficient).
deficit def·i·cit (děf'ĭ-sĭt)
A lack or deficiency of a substance.
A lack or impairment in mental or physical functioning.
A shortage, especially the amount by which a sum of money falls short of what is required; a debt.