This means in a sudden move or all at once but started out with more sinister connotations.

The expression is often wrongly quoted as ‘one foul swoop’, or even ‘one fowl swoop’, but it doesn’t relate to chickens.

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?

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) and this is the ‘fell’ of the quote.

So the Bard’s original words were penned to convey the idea 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.