In the phrase one fell swoop—which means suddenly or in a single, swift action—fell is an adjective meaning fierce, savage, cruel, or ruthless,1 and swoop is a noun meaning either (1) a blow or stroke or (2), metaphorically, a bird’s sudden, sweeping descent from a height upon its prey.2 We can imagine contexts where one foul swoop might make sense, but these must be rare.
The earliest documented instance of one fell swoop is from Shakespeare’s Macbeth (1605):
All my pretty ones?
Did you say all? O hell-kite! All?
What, all my pretty chickens and their dam
At one fell swoop?
Examples
One foul swoop occasionally appears in place of one fell swoop, sometimes with funny results—for example:
The reality is that many of the problems we talk about cannot be solved in one foul swoop. [The Courier]
The mess of a situation claimed the souls of eight Ferraris in one foul swoop. [Motor Authority]
And here are a few examples of one fell swoop used well:
The board could have listened patiently, collected its thoughts and forcefully addressed that hyperbolic message in one fell swoop. [New York Times]
The only winner was of course French president Nicolas Sarkozy, who achieved his dual agenda in one fell swoop. [Financial Times]
Waiting at the checkout may not be everyone’s idea of fun, but it’s a way to get your shopping sorted in one fell swoop. [The Age]
References
1. http://www.oed.com/view/Entry/69069 (subscription required) ^
2. http://www.oed.com/view/Entry/196025 (subscription required) ^
Other resources
“At one fell swoop” at The Phrase Finder
“One fell swoop” at World Wide Words
