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.
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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]