mid-15c., "an inhabitant, one who dwells in a place permanently or for a considerable time," from resident (adj.). Meaning "medical graduate doing supervised specialized practice in a hospital as training" is attested by 1892, American English.
late 14c., "dwelling, residing, having an abode in a place for a continuance of time," from Old French resident and directly from Latin residentem (nominative residens), present participle of residere "to sit down, settle" (see reside). From early 15c. as "stay in a place in discharge of some duty," originally ecclesiastical.