Is it farther or further?
mid-15c., from wind (n.) + fall (n.1). Originally literal, in reference to wood or fruit blown down by the wind, and thus free to all. Figurative sense of "unexpected acquisition" is recorded from 1540s.
An unexpected profit from a business or other source. The term connotes gaining huge profits without working for them — for example, when oil companies profit from a temporary scarcity of oil.