At one fell swoop
Suddenly; in a single action.
This is one of those phrases that we may have picked up early in our learning of the language and probably worked out its meaning from the context in which we heard it, without any clear understanding of what each word meant. Most native English speakers could say what it means but, if we look at it out of context, it doesn't appear to make a great deal of sense. That lack of understanding of the words in the phrase is undoubtedly the reason that this is often misspelled, for example, 'at one fail swoop', or even, with more justification as it might be thought to relate to birds, 'one fowl swoop'. It isn't difficult to also find examples of 'one foul swoop'. 'Stoop' is sometimes substitued for 'swoop' in all of the above variants, again drawing on avian imagery.
So, what's that 'fell'? We use the word in a variety of ways: to chop, as in fell a tree; a moorland or mountain, like those in the northern UK; the past tense of fall, as in 'he fell over'. None of those seems to make sense in this phrase and indeed the 'fell' here is none of those. It's an old word, in use by the 13th century, that's now fallen out of use other than in this phrase, and is the common root of the term 'felon'. The Oxford English Dictionary defines 'fell' as meaning 'fierce, savage; cruel, ruthless; dreadful, terrible', which is pretty unambiguous.
Shakespeare either coined the phrase, or gave it circulation, in Macbeth, 1605:
MACDUFF: [on hearing that his family and servants have all 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?
The kite referred to is a hunting bird, like the Red Kite, which was common in England in Tudor times and is now making a welcome return after near extinction in the 20th century. The swoop (or stoop as is sometimes now said) is the rapid descent made by the bird when capturing prey.
Shakespeare used the imagery of a hunting bird's 'fell swoop' to indicate the ruthless and deadly attack by Macbeth's agents.
In the intervening years we have rather lost the original meaning and use it now to convey suddenness rather than savagery.
See other phrases and sayings from Shakespeare.