When someone wins the lottery people often say he had a "windfall", meaning unexpected good luck.
The word was first used in 11th century England.
William the Conqueror, who ruled at the time, wanted to protect the forests so he made it a crime for peasants to cut down trees.
But the peasants needed wood to build homes and make fires.
So William agreed that they could use trees the wind had blown down.
In time, any tree that had been blown down was called a "windfall" or a stroke of good luck for some poor peasant. - english-test.net; bigsiteofamazingfacts.com; inyonline.co.uk