In plural. A flock of birds (usually wild geese) making a noise in the air, and imagined or represented as a gathering of supernatural creatures presaging death or misfortune or as the souls of children who have died unbaptized. In singular: the noise made by these birds or creatures.
Late 18th century; earliest use found in John Hutton (1739–1806), writer on topography and etymology. Partly (i) a variant or alteration of Gabriel ratchet.