At one fell swoop

This means all in one go or in one sudden action.

In Shakespeare’s Macbeth, Macduff on hearing of the death of his family says;
All my pretty ones?
Did you say all? O hell-kite! All?
What, all my pretty chickens and their dam
At one fell swoop?

This is often wrongly quoted as ‘one foul swoop’ or even ‘one fowl swoop’.

The swoop is the rapid descent of the kite but what of the fell? Fell has a few meanings including, as a noun a stretch of moorland, and as a verb to cut down. It also has a meaning as an adjective  ‘of terrible evil or ferocity’ (Oxford Dictionary of English) and this is the ‘fell’ of the quote.

In this image of the ferocious swoop of the hunting bird to snatch its prey, Shakespeare creates a powerful metaphor for the sudden and brutal killing of Macduff’s kin.

The phrase now doesn’t usually have the meaning of savagery, just suddenness.

There is another meaning of fell not heard anymore, ‘an animal’s hide or skin with its hair’. A fellmonger was a dealer in the skins.