The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]WordNet (r) 3.0 (2006):
unfailingly adv 1: without fail