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Adjective: unfailing  ,ún'fey-ling
  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