The Collaborative International Dictionary of English v.0.48:

Slip \Slip\, v. i. [imp. & p. p. Slipped; p. pr. & vb. n.
   Slipping.] [OE. slippen; akin to LG. & D. slippen, MHG.
   slipfen (cf. Dan. slippe, Sw. slippa, Icel. sleppa), and fr.
   OE. slipen, AS. sl[imac]pan (in comp.), akin to G. schleifen
   to slide, glide, drag, whet, OHG. sl[imac]fan to slide,
   glide, make smooth, Icel. sl[imac]pa to whet; cf. also AS.
   sl?pan, Goth. sliupan, OS. slopian, OHG. sliofan, G.
   schliefen, schl?pfen, which seem to come from a somewhat
   different root form. Cf. Slope, n.]
   1. To move along the surface of a thing without bounding,
      rolling, or stepping; to slide; to glide.
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   2. To slide; to lose one's footing or one's hold; not to
      tread firmly; as, it is necessary to walk carefully lest
      the foot should slip.
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   3. To move or fly (out of place); to shoot; -- often with
      out, off, etc.; as, a bone may slip out of its place.
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   4. To depart, withdraw, enter, appear, intrude, or escape as
      if by sliding; to go or come in a quiet, furtive manner;
      as, some errors slipped into the work.
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            Thus one tradesman slips away,
            To give his partner fairer play.      --Prior.
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            Thrice the flitting shadow slipped away. --Dryden.
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   5. To err; to fall into error or fault.
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            There is one that slippeth in his speech, but not
            from his heart.                       --Ecclus. xix.
                                                  16.
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   To let slip, to loose from the slip or noose, as a hound;
      to allow to escape.
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            Cry, "Havoc," and let slip the dogs of war. --Shak.
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WordNet (r) 3.0 (2006):

slipping
    adj 1: moving as on a slippery surface; "his slipping and
           slithering progress over the ice" [syn: slipping,
           slithering]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "slipping":
   bad, coming apart, cracking, crumbling, decadent, declining,
   degenerate, despaired of, deteriorating, disintegrating, done for,
   draining, drooping, dwindling, dying, ebbing, effete, expiring,
   facing death, fading, failing, falling, flagging, fragmenting,
   given up, going, going to pieces, gone to seed, hopeless,
   in articulo mortis, in extremis, incapable of life, languishing,
   low, marcescent, moribund, near death, nonviable, on the downgrade,
   out of condition, out of shape, out of training, pining,
   regressive, retrograde, retrogressive, run to seed, rusty,
   shriveling, sinking, sliding, slipping away, slumping, soft, stiff,
   subsiding, tabetic, terminal, waning, wasting, wilting, withering,
   worsening