The Collaborative International Dictionary of English v.0.48:

Overconfidence \O"ver*con"fi*dence\, n.
   Excessive confidence; too great reliance or trust.
   [1913 Webster]

WordNet (r) 3.0 (2006):

overconfidence
    n 1: total certainty or greater certainty than circumstances
         warrant [syn: certitude, cocksureness,
         overconfidence]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "overconfidence":
   arrogance, assurance, assuredness, belief, brashness, brass,
   brazen boldness, certitude, cheek, chutzpah, cocksureness,
   confidence, confidentness, conviction, courage, faith, gall,
   heroics, hubris, improvidence, imprudence, impudence, indiscretion,
   injudiciousness, insolence, overboldness, overcarelessness,
   oversureness, overweening, overweeningness, poise, pomposity,
   positiveness, pride, rashness, security, self-assurance,
   self-confidence, self-importance, self-reliance, settled belief,
   subjective certainty, sureness, surety, temerariousness, temerity,
   trust, unchariness, unwariness