The Collaborative International Dictionary of English v.0.48:

Slump \Slump\, v. i. [imp. & p. p. Slumped; p. pr. & vb. n.
   Slumping.] [Scot. slump a dull noise produced by something
   falling into a hole, a marsh, a swamp.]
   1. To fall or sink suddenly through or in, when walking on a
      surface, as on thawing snow or ice, partly frozen ground,
      a bog, etc., not strong enough to bear the person.
      [1913 Webster]

            The latter walk on a bottomless quag, into which
            unawares they may slump.              --Barrow.
      [1913 Webster]

   2. To slide or slip on a declivity, so that the motion is
      perceptible; -- said of masses of earth or rock.
      [Webster 1913 Suppl.]

   3. To undergo a slump, or sudden decline or falling off; as,
      the stock slumped ten points. [Colloq.]
      [Webster 1913 Suppl.]