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]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "fallout":
   A-blast, A-bomb shelter, H-blast, Mach front, activity,
   air pollution, airborne radioactivity, artificial radioactivity,
   atom blast, attritus, blast wave, bran, contamination, cosmic dust,
   crumb, crumble, curiage, decontamination, dust, dust ball,
   dust cloud, efflorescence, fallout shelter, farina, filings,
   fission particles, flash burn, flour, grits, groats, ground zero,
   half-life, hydrogen blast, kittens, lint, meal, mushroom cloud,
   natural radioactivity, nuclear radiation, powder, pussies,
   radiant energy, radiation, radioactive dust, radioactive radiation,
   radioactivity, radiocarbon dating, radiolucency, radiopacity,
   radiosensibility, radiosensitivity, raspings, saturation point,
   sawdust, smut, soot, specific activity