Hit a patch of Lord of the Rings where several things were described as fell, so I decided to look it up to see where the word came from. This is what my iPhone Oxford said:
fell
adjective
poetic/literary
of terrible evil or ferocity; deadly.
phrases:
in or at one fell swoop– all at one time. [origin: from Shakespeare’s Macbeth( iv. iii. 219).]
etymology:
Middle English: from Old French fel, nominative of felon wicked (person) (see felon).
I read this and thought how could I have not linked fel with fell swoop? But now I know and so do you!
It makes me thing of an eagle or owl, swooping down…a “fell deed” to whatever they plan to eat!