A FOWL is a bird, like a pheasant, a chicken or a duck.
One tends to picture a Nineteenth Century English countryside with English gentlemen blasting the skies with their shotguns, hounds yapping at their heels eager to collect dead birds; That or fond memories of childhood. See Danny the Champion of the World.
They are not a swooping bird so much as a bird that gets swatted at.
An adjective meaning abhorrent or disgusting.
So we have;
- A foul deed;
- A foul stench; or,
- A foul play;
From where we get the noun, foul, meaning, in turn:
- To make shocking to the mind. As in, we all thought the bloody murder was foul;
- To be smelly, or on the nose. As in, your armpits are foul, go wash them; or,
- An illegal or unfair play. As in, the batter stuck the ball but it went off field and was a foul.
- Fell is the past tense of fall, sure, as is, I fell on my arse and broke my tail bone;
- It is also the verb, to fell, that is causing something to fall, as in, the lumberjack fell the tree causing it to fall;
- But perhaps most interestingly, it is an adjective meaning dangerous, wicked, cruel, or deadly, as in, the murderer spiced my drink with fell poison, upon which I did fall to the ground.This last meaning is little used and can most clearly be thought of as the nominative of felon, ie. when naming something particularly felonious. So for example we might say, of the felons that escaped prison, only one, a notoriously fell man, remained at large.
It is in this final sense that we get the phrase, one fell swoop, meaning quickly, suddenly and dangerously so.
Like a lot of these historical phrases of uncertain origin, Shakespeare is a likely candidate.
In a particularly memorable and heartfelt moment, Macduff upon hearing that Macbeth has had his children and his wife killed, cries:
“All my pretty ones?
Did you say all? — O, hell-kite! All?
What, all my pretty chickens and their dam
At one fell swoop?”
It is some startling imagery. That Macbeth like a hell hawk has swooped down and snatched Macduff’s children (the chickens) and their dam (their demoiselle, or damsel, their mother).
Perhaps this imagery is how the confusion occurs.
A fowl swoop seems only natural, that a fowl might swoop, and indeed Macbeth’s fell swoop is in imagery at least a fowl one, it is also entirely conceivably a foul one.
Further I supposed we could imagine a foul fowl swoop, say a magpie attacking you from behind .
Yes, there is nothing wrong with saying a foul swoop: a particularly horrid swoop; or fowl swoop, being the swoop made by a pheasant, however I would absolutely avoid them even when you mean exactly that, and use only fell swoop to mean something that happens dangerously and wickedly, for that is it’s meaning.
Perhaps even quote a bit of Macbeth.