What is the X in X-mas?
late 13c., from Old French contrefait "imitated" (Modern French contrefait), past participle of contrefaire "imitate," from contre- "against" (see contra-) + faire "to make, to do" (from Latin facere; see factitious). Medieval Latin contrafactio meant "setting in opposition or contrast." Related: Counterfeited; counterfeiting. The noun and adjective are from late 14c.