What is the X in X-mas?
early 15c., from Middle French defect and directly from Latin defectus "failure, revolt, falling away," noun use of past participle of deficere "to fail, desert" (see deficient).
1570s, from Latin defectus, past participle of deficere "to fail, desert" (see defect (n.)). Related: Defected; defecting.
defect de·fect (dē'fěkt', dĭ-fěkt')
A lack of or abnormality in something necessary for normal functioning; a deficiency or imperfection.