Source : Webster's Revised Unabridged Dictionary (1913)

Unfailing \Un*fail"ing\, a.
   Not failing; not liable to fail; inexhaustible; certain;
   sure. --Dryden. -- {Un*fail"ing*ly}, adv. --
   {Un*fail"ing*ness}, n.

Source : WordNet®

unfailing
     adj 1: not liable to failure; "a foolproof identification system";
            "the unfailing sign of an amateur"; "an unfailing
            test" [syn: {foolproof}]
     2: always able to supply more; "an unfailing source of good
        stories"; "a subject of unfailing interest"
     3: unceasing; "unfailing loyalty"; "unfailing good spirits";
        "unflagging courtesy" [syn: {unflagging}]