WordWeb Online

1. Not liable to failure
   "the unfailing sign of an amateur"; "an unfailing test";
   - foolproof, idiot-proof [informal]
    
2. Always able to supply more
   "an unfailing source of good stories"; "a subject of unfailing interest"
    
3. Tireless
   "unfailing loyalty";
   - unflagging

See also: constant, inexhaustible, infallible