"Suddenly, in a single action"
I started thinking about this phrase today because I realized I didn't know how to spell the second word. I quickly discovered that the main reason I didn't know how to spell it was because I was mispronouncing it. I always thought it was one foul/fowl swoop. Egg on my face.
So now that we know the correct way to say it (good for you if you've known it all along, smarty pants), the question is: where does it come from?
It is first attested by Shakespeare in 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?
The fact that Shakespeare was the first to write it down (that we know of) means one of two things: he coined it or it was an existing slang/idiomatic phrase. Shakespeare is the first attestation of many English words because he wrote very idiomatically, which was a revolutionary idea in his day. He also coined his share of words, but it can be hard to tease out what he invented and what he took from slang of various language groups around him.
Shakespeare's usage of one fell swoop gives up clues as to the origin of the meaning. A kite is a hunting bird, which swoops down to catch its prey. Fell in this context means something different than what Modern English speakers might expect: "fierce, savage, cruel." Over the centuries the sense of "savageness" was lost and we are left with a meaning of "all at once."