The phrase at (or in) one fell swoop means all in one go. Here, the noun swoop, which denotes the act of swooping down, refers to the sudden pouncing of a bird of prey (a kite for example) from a height upon its quarry (of Germanic origin, the verb swoop is cognate with sweep). The adjective fell (related to felon) means of terrible evil or ferocity. The whole expression is from The Tragedie of Macbeth (around 1603), by the English poet and playwright William Shakespeare (1564-1616); Macduff is struggling to come to terms with the murder of his children and wife on the orders of the “hell-kite” Macbeth:
(Folio 1, 1623)
– Ross: Your Castle is surpriz’d: your Wife, and Babes
Sauagely slaughter’d: To relate the manner
Were on the Quarry of these murther’d Deere
To adde the death of you.
– Malcolm: Mercifull Heauen:
What man, ne’re pull your hat vpon your browes:
Giue sorrow words; the griefe that do’s not speake,
Whispers the o’re-fraught heart, and bids it breake.
– Macduff: My Children too?
– Ross: Wife, Children, Seruants, all that could be found.
– Macduff: And I must be from thence? My wife kil’d too?
– Ross: I haue said.
– Malcolm: Be comforted.
Let’s make vs Med’cines of our great Reuenge,
To cure this deadly greefe.
– Macduff: He ha’s no Children. All my pretty ones?
Did you say All? Oh Hell-Kite! All?
What, All my pretty Chickens, and their Damme
At one fell swoope?
The earliest use of one fell swoop, without explicit reference to Shakespeare, that I could find is in a letter published by The Chester Chronicle (Cheshire) of Friday 16th May 1800:
The French people are now convinced of the sincerity of Bonaparte, and as they have swallowed his anodyne, they will support him on the throne, and there is no doubt that the stimulus will also operate. Had our Ministers possessed the sagacity of the Lover of his Country*, how easily they might have converted the anodyne into poison, the stimulus into a repellent, and, by one fell swoop, got rid of their enemy, and restored peace to England!
(* ‘A Lover of his Country’ had written a letter to this newspaper.)
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