The origin of the meaning of the word windfall seems ambiguous. It was something blown down by the wind – sounds like something nasty (a tree, a house) – but turns out it is overwhelmingly a good thing, although in economics, for example, there is the term windfall loss, which is not a good thing.<2017 “There are legitimate reasons to run deficits, including lifting the economy in tough times, strengthening the military against proven threats and building or rebuilding public infrastructure. Borrowing trillions of dollars to provide a huge windfall for people at the top is not one of those reasons.”–New York Times, 27 April>
1) An unexpected gain, piece of good fortune, or the like.
2) Something blown down by the wind, as fruit. [[Seems like this could be good or bad: a) good if it saves picking. b) bad if it is prior to picking time and fruit rots on the ground.]]
Accruing in unexpectedly large amounts: windfall profits.
(Random House Webster’s Unabridged Dictionary)
The following quotes are from the Oxford English Dictionary and archived sources:
___________________________<1504 “They which fall before the time of gathering, as wind-falles.”—Fruiterers Secrets, by N.F., page 12> [bad thing if they rot on the ground]
<1661 “The wind begins to bluster among the Apples,..and the wind-falls are gathered to fill the Pies for the houshold.”––Twelve Moneths by M. Stevenson, page 42> [[good thing]]
<1801 “He . . . kept little windfalls, that came to him by the negligence of customers—tooth-pick cases, loose silver”—Forester in Moral Talesby M. Edgeworth, I. page 190>
<1936 “The change in the value of the equipment, due to unforeseen changes in market values, exceptional obsolescence or destruction . . . may be called the windfall loss.”—General Theory of Employment by J. M. Keynes, II. vi. page 57>
<1977 “The shift to free market pricing would give the oil companies windfall profits.”—N.Y. Review of Books, 26 May, page 31/4>
<1996 “Republicans such as presidential candidate Steve Forbes and House Majority Leader Dick Armey of Texas - would impose ‘a windfall loss’ on millions of American homeowners, . . .”—Rocky Mountain News (Denver, CO), 17 February>
<2017 “. . . is the fall in wind power tariffs sustainable or driven purely by the need to compete with solar power players, who are reaping the windfall of falling solar panel prices?”—Business Today, 7 May>
Ken Greenwald, April 27, 2017