One fell swoop

What is the origin and original meaning of the common phrase "one fell swoop"? "Fell"? Thanks.

A fell swoop is a sudden, complete and often unexpected event; over and done with great speed and totality. Why fell? This comes from the 13th century Old French word "fel" meaning "cruel". The speed of a fell swoop is so great as to be regarded as cruel by some. Our word Felon comes from the same source.

It was my mother's whimsy to make it "one swell foop," which amuses me still, 50 years later. Whatever. From a previous discussion:

The phrase stems from a passage in Macbeth. Macduff has just been told that his wife and children have been killed:

All my pretty ones?
Did you say all? O hell-kite! All?

What, all my pretty chickens and their dam

At one fell swoop?

(Macbeth IV.iii.216ff)