The Collaborative International Dictionary of English v.0.48:

Desperado \Des`per*a"do\, n.; pl. Desperadoes. [OSp.
   desperado, p. p. of desperar, fr. L. desperare. See
   Desperate.]
   A reckless, furious man; a person urged by furious passions,
   and regardless of consequence; a wild ruffian.
   [1913 Webster]

WordNet (r) 3.0 (2006):

desperado
    n 1: a bold outlaw (especially on the American frontier) [syn:
         desperado, desperate criminal]

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "desperado":
   Cain, Judas, apache, assassin, assassinator, badman, bandit,
   betrayer, bloodletter, bloodshedder, bravo, bucko, bully, bullyboy,
   burker, butcher, button man, cannibal, convict, criminal, crook,
   cutthroat, deceiver, desperate criminal, devil, double-dealer,
   eradicator, executioner, exterminator, felon, fugitive,
   gallows bird, gangster, gaolbird, garroter, gorilla, gun, gunman,
   gunsel, hatchet man, head-hunter, hell-raiser, hellcat, hit man,
   homicidal maniac, homicide, jailbird, killer, lawbreaker, mad dog,
   man-eater, man-killer, manslayer, massacrer, matador, mobster,
   murderer, outlaw, pesticide, poison, poisoner, public enemy,
   quisling, racketeer, rough, rowdy, ruffian, scofflaw, slaughterer,
   slayer, strangler, swindler, thief, thug, torpedo, traitor,
   trigger man, two-timer