The Collaborative International Dictionary of English v.0.48:

Windfall \Wind"fall`\, n.
   1. Anything blown down or off by the wind, as fruit from a
      tree, or the tree itself, or a portion of a forest
      prostrated by a violent wind, etc. "They became a windfall
      upon the sudden." --Bacon.
      [1913 Webster]

   2. An unexpected legacy, or other gain.
      [1913 Webster]

            He had a mighty windfall out of doubt. --B. Jonson.
      [1913 Webster]
      [1913 Webster]

WordNet (r) 3.0 (2006):

windfall
    n 1: fruit that has fallen from the tree
    2: a sudden happening that brings good fortune (as a sudden
       opportunity to make money); "the demand for testing has
       created a boom for those unregulated laboratories where boxes
       of specimen jars are processed like an assembly line" [syn:
       boom, bonanza, gold rush, gravy, godsend, manna
       from heaven, windfall, bunce]