means An unexpected piece of good fortune, especially a sudden gain of money you didn't earn or plan for.

from Literally fruit or branches blown down from trees by the wind. In old English forests, much standing timber belonged to the crown or the landlord, but wood the wind knocked loose was fair game for ordinary folk to gather — free fuel and lumber that nobody had to fell or pay for. From that gift of weather the word stretched, by the 1500s, to mean any unlooked-for stroke of luck dropping into your hands.