People have been using the phrase “at one fell swoop” in English since the 1600s, and like many idioms, many people are entirely unaware of its origins. This phrase is generally used to mean “all at once,” in a very rapid and final sense, although one could be forgiven for wondering what falling and swooping have to do with something happening suddenly and perhaps violently. Unlike many idioms, which seem to have appeared in the English language with no apparent origins, we actually do know where “one fell swoop comes from.”
To understand the origins of this phrase, we are going to need to read a little Shakespeare, because the first documented use of it appeared in the play Macbeth:
In Shakespeare's “one fell swoop,” a single incident changes the life of the character MacDuff forever; his entire family is murdered on the orders of Macbeth, who fears that MacDuff is angling for his throne. In the end, Macbeth's fears turn out to be well-grounded, as he is ultimately defeated by MacDuff at the end of the play.
The root for “fell” in this sense is the French fel, which means “evil.” Although we no longer use the word in this sense, except in obscure poetry, we do retain another word in the English language with this root: “felon.” Fell as in “to fall” comes from an entirely different Anglo-Saxon word, illustrating the diverse roots of the English language.
People often use this phrase to describe the accomplishment of several tasks with a single action, as in “the candidate rearranged the campaign staff at one fell swoop.” The term implies finality and rapidity, sometimes with a hint brutal force. It is also sometimes mispronounced as “one swell foop,” sometimes deliberately by people who want to bring levity to a serious situation.