What is the X in X-mas?
mid-15c., "an inhabitant, one who resides," from resident (adj.). Meaning "medical graduate in practice in a hospital as training" first attested 1892, American English.
late 14c., "dwelling, residing," from Old French resident and directly from Latin residentem (nominative residens), present participle of residere "to sit down, settle" (see reside).
resident res·i·dent (rěz'ĭ-dənt, -děnt')
A physician during residency.