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I knew the saying is 'one fell swoop' Bigglesworth but I thought I'd look it up just to see what it meant exactly as I could never rationalise the link I guessed had to exist between fell as in 'felling' a tree and 'one fell swoop'.
Answer below is a bit of a surprise as there is an indirect link to birds or 'fowl' which one may expect with the word swoop involved:

It's quite common for people to use the phrase "one fowl swoop" (or even one foul swoop) when they want to convey the idea of an event taking place all at once and very suddenly. But why do we say this? Is the phrase something to do with birds swooping to the ground in a great rush?

In fact the phrase does have some connection to birds but both "one fowl swoop" and "one foul swoop" are incorrect. The original phrase is actually "one fell swoop".

The phrase is an old one. It may have been coined by Shakespeare in 1605, or he may merely have popularised it. In any case, the following lines can be found in Macbeth (Act 4, scene 3). Macduff has just heard that his family have been killed:

All my pretty ones?
Did you say all?- O hell-kite - All?
What, all my pretty chickens, and their dam,
At one fell swoop?

So Shakespeare does employ the image of a bird of prey, a "hell-kite", swooping to the ground to kill all his "pretty chickens". But the word he uses is "fell". This, of course, is still a common word, but Shakespeare is using a rather obscure sense of that word, meaning of terrible evil or ferocity. Its source is the same as felon, a wicked person.

So, "one fell swoop" originally meant a sudden, ferocious attack, although the sense of savagery in the phrase has been lost over the years and people now use it to mean, simply, all at once.

Footnote : the dive of a bird of prey is more accurately referred to as a stoop rather than a swoop as in, for example, "the peregrine falcon can reach speeds of over 200 mph in a stoop".