The Collaborative International Dictionary of English v.0.48:

Slippery \Slip"per*y\, a. [See Slipper, a.]
   1. Having the quality opposite to adhesiveness; allowing or
      causing anything to slip or move smoothly, rapidly, and
      easily upon the surface; smooth; glib; as, oily substances
      render things slippery.
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   2. Not affording firm ground for confidence; as, a slippery
      promise.
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            The slippery tops of human state.     --Cowley.
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   3. Not easily held; liable or apt to slip away.
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            The slippery god will try to loose his hold.
                                                  --Dryden.
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   4. Liable to slip; not standing firm. --Shak.
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   5. Unstable; changeable; mutable; uncertain; inconstant;
      fickle. "The slippery state of kings." --Denham.
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   6. Uncertain in effect. --L'Estrange.
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   7. Wanton; unchaste; loose in morals. --Shak.
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   Slippery elm. (Bot.)
      (a) An American tree (Ulmus fulva) with a mucilagenous
          and slightly aromatic inner bark which is sometimes
          used medicinally; also, the inner bark itself.
      (b) A malvaceous shrub (Fremontia Californica); -- so
          called on the Pacific coast.
          [1913 Webster]

WordNet (r) 3.0 (2006):

slippery
    adj 1: causing or tending to cause things to slip or slide;
           "slippery sidewalks"; "a slippery bar of soap"; "the
           streets are still slippy from the rain" [syn: slippery,
           slippy] [ant: nonslippery]
    2: not to be trusted; "how extraordinarily slippery a liar the
       camera is"- James Agee [syn: slippery, tricky]

Moby Thesaurus II by Grady Ward, 1.0:

186 Moby Thesaurus words for "slippery":
   Machiavellian, Machiavellic, acute, adipose, ambidextrous, amoral,
   arch, artful, astute, blubbery, buttery, butyraceous, cagey,
   calculating, canny, changeable, chiseling, chrismal, chrismatory,
   clever, collusive, conscienceless, corrupt, corrupted, covinous,
   crafty, criminal, critical, crooked, cunning, cute, dangerous,
   dark, deceitful, deep, deep-laid, delicate, designing, desultory,
   devious, diplomatic, dishonest, dishonorable, disloyal, double,
   double-dealing, doubtful, dubious, elusive, elusory, evasive,
   faithless, false, falsehearted, fat, fatty, feline, felonious,
   finagling, fishy, foxy, fraudulent, furtive, glassy, greased,
   greasy, guileful, hazardous, icy, ill-got, ill-gotten, immoral,
   inconstant, indirect, infirm, ingenious, insecure, insidious,
   insubstantial, inventive, knowing, lardaceous, lardy, lubric,
   lubricated, lubricious, malingering, mucoid, not kosher, oiled,
   oily, oleaginous, oleic, on slippery ground, on thin ice, pawky,
   perfidious, perilous, politic, precarious, provisional,
   questionable, ready, reptilian, resourceful, rich, risky, rotten,
   scheming, sebaceous, serpentine, shady, shaky, shameless, sharp,
   shifting, shifty, shirking, shrewd, sinister, sleek, slick,
   sliddery, slimy, slipper, slippy, slithery, sly, smooth, snaky,
   sneaky, soaped, soapy, sophistical, stealthy, strategic, subtile,
   subtle, suety, supple, surreptitious, suspicious, tactical,
   tallowy, temporary, tentative, ticklish, touch-and-go, touchy,
   treacherous, trickish, tricksy, tricky, two-faced, uncertain,
   unconscienced, unconscientious, unconscionable, unctuous,
   undependable, underhand, underhanded, unethical, unfaithworthy,
   unguent, unguentary, unguentous, unprincipled, unreliable,
   unsavory, unscrupulous, unsolid, unsound, unstable, unsteadfast,
   unsteady, unstraightforward, unsubstantial, unsure, untrustworthy,
   variable, vulpine, wary, wily, without remorse, without shame