The Collaborative International Dictionary of English v.0.48:

Underhanded \Un"der*hand`ed\, a.
   1. Underhand; clandestine.
      [1913 Webster]

   2. Insufficiently provided with hands or workers;
      short-handed; sparsely populated; obsolete in this sense,
      short-handed or understaffed being the preferrred
      term.
      [1913 Webster +JG]

            Norway . . . might defy the world, . . . but it is
            much underhanded now.                 --Coleridge.
      [1913 Webster]

WordNet (r) 3.0 (2006):

underhanded
    adj 1: marked by deception; "achieved success in business only
           by underhand methods" [syn: sneaky, underhand,
           underhanded]
    2: with hand brought forward and up from below shoulder level;
       "an underhand pitch"; "an underhand stroke" [syn:
       underhand, underhanded, underarm] [ant: overarm,
       overhand, overhanded]

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "underhanded":
   amoral, artful, back-door, backstairs, calculating, chiseling,
   clandestine, collusive, conscienceless, corrupt, corrupted, covert,
   covinous, crafty, criminal, crooked, cunning, dark, deceitful,
   devious, dishonest, dishonorable, doubtful, dubious, duplicitous,
   evasive, false, falsehearted, feline, felonious, finagling, fishy,
   fraudulent, furtive, guileful, hidlings, hole-and-corner,
   hugger-mugger, ill-got, ill-gotten, immoral, indirect, insidious,
   not kosher, privy, questionable, quiet, rotten, scheming, shady,
   shameless, sharp, shifty, sinister, skulking, slinking, slinky,
   slippery, sly, sneaking, sneaky, stealthy, surreptitious,
   suspicious, treacherous, trickish, tricky, two-faced,
   unconscienced, unconscientious, unconscionable, under-the-counter,
   under-the-table, undercover, underground, underhand, undermanned,
   understaffed, unethical, unobtrusive, unprincipled, unsavory,
   unscrupulous, unstraightforward, wily, without remorse,
   without shame