Let's make us med'cines of our great revenge
To cure this deadly grief.
Macduff:Macbeth Act 4, scene 3, 213–219
He has no children.—All my pretty ones?
Did you say all?—O hell-kite!—All?
What, all my pretty chickens, and their dam,
At one fell swoop?
We employ "fell" almost exclusively as the past tense of "fall," and occasionally as a verb in its own right, meaning "to cut down." Macduff uses "fell" in a sense that is now rare—as an adjective meaning "fierce, deadly." King Macbeth, who knows that Macduff is conspiring to overthrow him, had ordered the murder of Macduff's wife, children, and servants. This is the "fell swoop": Macduff likens Macbeth to a "hell-kite" (the kite is a vicious bird of prey in the falcon family). While Macduff's poultry metaphor has not worn well, the phrase "one fell swoop" has become a regular part of the language. Part of its appeal, besides its evocative sound, is its economic phrasing, which suggests all three meanings of "fell": Macbeth figuratively "falls" from the sky in a "fierce, deadly" swoop to "cut down" Macduff's family.