Fell, from Old English, means awful, terrible or horrible. The word’s stem can also be seen in “felon,” which now is mostly used to mean someone who has been convicted of a felony (a serious crime), but which formerly meant one who is terrible, horrible or awful in behavior. The “swoop” is an onomotopoeia, indicating a fast movement. All together, “one fell swoop” means a swift, horrible blow.
Shakespeare, originator of so many English catchphrases, may have dreamed up this one too. It appears in “Macbeth”: “What! all my pretty chickens and their dam/At one fell swoop?” (act IV, scene 3) laments Macduff, upon learning his wife and children have been killed by Macbeth. This appears to be the earliest recorded use of the phrase, although it may have been in common usage before Shakespeare wrote it down.
It’s interesting that “one fell swoop,” which originally had such a dire connotation, is now a mild term meaning “all at once.”
So, while I’ve admittedly not pulled out my massive OED (we have the full 12 volumes bound as one with microscopic type), I’m pretty sure I’ve learned something new tonight.
Until another day-
[FN] I know, I know. It sounds mad sketchy. But I did my fair share of other googling on this and this was the best overview of everything else I’d read.