Orchardists may think of apples, but windfall was first applied to trees. One of the most pressing problems for landless English peasants was the shortage of fuel. Medieval forests were thick and abundant, but a royal proclamation forbade commoners to fell a tree without permission. Only dead branches and blowdowns could be gathered for firewood.
Consequently, a windfall, or the discovery of fallen timber, was a legitimate and grateful find, sometimes making the difference between a blazing hearth and a long, bitter-cold night. From trees felled by the wind came this expression meaning any piece of good fortune.
The earliest meaning for this common nautical term is horn player, coined around 1870 from the German word for wind and the verb jammer, meaning to moan or cry. A horn player makes low, wailing sounds with his wind instrument, and - no surprise - in time a windjammer was a talkative person, or windbag.
By the end of the 19th century, the term also applied to any ship with sails. Why? It seems that the defenders of sailing vessels during the early age of steamships boasted so much about the merits of sail over steam that their steaming rivals tacked this term onto them and eventually onto their ships.
SOURCES: 'Horsefeathers,' by C. E. Funk; 'The Encyclopedia of Word and Phrase Origins,' by Robert Hendrickson; 'Why You Say It,' by Webb Garrison; 'Dictionary of Word Origins,' by Joseph Shipley.