The Collaborative International Dictionary of English v.0.48:

Temerity \Te*mer"i*ty\, n. [L. temeritas, from temere by chance,
   rashly; perhaps akin to Skr. tamas darkness: cf. F.
   t['e]m['e]rit['e].]
   Unreasonable contempt of danger; extreme venturesomeness;
   rashness; as, the temerity of a commander in war.
   [1913 Webster]

   Syn: Rashness; precipitancy; heedlessness; venturesomeness.

   Usage: Temerity, Rashness. These words are closely allied
          in sense, but have a slight difference in their use
          and application. Temerity is Latin, and rashness is
          Anglo-Saxon. As in many such cases, the Latin term is
          more select and dignified; the Anglo-Saxon more
          familiar and energetic. We show temerity in hasty
          decisions, and the conduct to which they lead. We show
          rashness in particular actions, as dictated by sudden
          impulse. It is an exhibition of temerity to approach
          the verge of a precipice; it is an act of rashness to
          jump into a river without being able to swim.
          Temerity, then, is an unreasonable contempt of danger;
          rashness is a rushing into danger from thoughtlessness
          or excited feeling.
          [1913 Webster]

                It is notorious temerity to pass sentence upon
                grounds uncapable of evidence.    --Barrow.
          [1913 Webster]

                Her rush hand in evil hour
                Forth reaching to the fruit, she plucked, she
                eat.                              --Milton.
          [1913 Webster]

WordNet (r) 3.0 (2006):

temerity
    n 1: fearless daring [syn: audacity, audaciousness,
         temerity]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "temerity":
   assurance, audacity, brashness, brass, brazen boldness, cheek,
   chutzpah, daring, foolhardiness, gall, hardihood, hardiness,
   heedlessness, heroics, hubris, impertinence, impetuosity,
   improvidence, imprudence, impudence, indiscretion, injudiciousness,
   insolence, intrusiveness, nerve, overboldness, overcarelessness,
   overconfidence, oversureness, overweeningness, precipitateness,
   rashness, recklessness, temerariousness, unchariness, unwariness,
   venturesomeness