Which is correct? Any insight on its origins?
The phrase comes from Shakespeare’s Macbeth – Macduff has just been told that while he’s been out of the country Macbeth has had his entire family killed:
He has no children. All my pretty ones?
Did you say all? O hell-kite! All?
What, all my pretty chickens and their dam
At one fell swoop?
One fell swoop.
From Macbeth, as Katisha noted.
“Fell” in the sense of cruel and destructive.
“Swoop” in the sense of a bird of prey descending on its prey with closed wings – i.e. swiftly.
Here is a very old thread on the subject.