The Collaborative International Dictionary of English v.0.48:

fallout \fallout\ n.
   1. the radioactive particles that settle to the ground after
      a nuclear explosion.

   Syn: radioactive dust, radioactive fallout.
        [WordNet 1.5]

   2. the falling to the ground of radioactive particles lifted
      into the atmosphere by a nuclear explosion.
      [PJC]

   3. an incidental or unexpected effect, especially one which
      is undesirable, consequent to an event or process;; --
      usually used only in the singular; as, the fallout from
      the disclosure of the Lewinsky tapes made trouble for the
      President for months after the event; fallout from the
      stock market crash caused property prices to decline in
      the New York area.
      [PJC]

   4. (Med.) one selected from a group by some criterion. [cant]
      [PJC]

            Corrective action was taken in 97 of the 418
                                                  --fallouts
            from 3,787 patients at risk.          --H. Gill
                                                  Cryer et al.
                                                  (Journal of
                                                  Trauma,
                                                  Injury,
                                                  Infection, and
                                                  Critical Care.
                                                  Vol 41, no. 3,
                                                  1996).
      [PJC]

   5. one who fails to maintain the same pace as and lags behind
      a group of which s/he is a member. [cant]
      [PJC]

            The executive officer's group noted all --fallouts
            by name and policed them into a group to complete
            the run at a slower pace.             --Lt. Col.
                                                  William C.
                                                  David
                                                  (Preparing a
                                                  Battalion for
                                                  Combat:
                                                  Physical
                                                  Fitness and
                                                  Mental
                                                  Toughness.
                                                  Army
                                                  University
                                                  After Next -
                                                  Virtual
                                                  Research
                                                  Library).
      [PJC]

WordNet (r) 3.0 (2006):

fallout
    n 1: the radioactive particles that settle to the ground after a
         nuclear explosion [syn: fallout, radioactive dust]
    2: any adverse and unwanted secondary effect; "a strategy to
       contain the fallout from the accounting scandal" [syn: side
       effect, fallout]