The definition of "windfall" as a unforseen gain actually came about as a combination of definitions. The gain mentioned actually *did* come from the fallen fruit extension. For a goodly time, it was of course stealing to take apples or other fruit from orchards belonging to anyone else, and this crime would be prosecuted. However, under an unspoken rule, any fruit that *fell* from the tree naturally, and was on the ground, was free for the taking. So whenever wind (unpredictably) knocked down crops, anyone could take the profit for themselves.