From where did the phrase ‘one fell swoop’ originate?
Grammarist.com notes that ‘fell’ in this phrase is an adjective meaning fierce, savage, cruel, or ruthless. This sense of fell is otherwise archaic, preserved mainly in this idiom. The swoop in one fell swoop is a noun referring to (1) a blow or stroke or (2), metaphorically, a bird’s sudden, sweeping descent from a height.
The earliest documented instance of one fell swoop is from Shakespeare’s 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?
Shakespeare clearly means swoop in its second sense (i.e., a bird’s sudden, swift descent), but the phrase is sometimes used with swoop being closer to its first meaning.