For all intensive purposes
Johnson’s wry note suggested replacing “one fell swoop” with “one foul swoop”. Macduff wails in Macbeth, “Did you say all? O hell-kite! All? What, all my pretty chickens and their dam, At one fell swoop?” Shakespeare is using an avian metaphor to compare the murder of Macduff’s wife and children to a hawk suddenly swooping down on defenceless prey. Surely “one fowl swoop” is to be preferred?
Professor Michael Mainelli