I always hear "in one fell swoop"

I knew that "fell" as an adjective meant "terrible" (exposure to a computer RPG that had a "fell beast" as an enemy type helped); so I kind of guessed that if you do everything "in one fell swoop" it means with one terrible, indiscriminate strike.

"Fell beast" is a phrase J. R. R Tolkien used in The Lord of the Rings [1], probably owing to his study of medieval literature. The more current usage may have stemmed from reading his works. [1] http://tolkiengateway.net/wiki/Fell_beasts

Did you not read the article?

It is clearly Shakespeare who popularized - if not outright invented - the term.

Shakespeare owns the English language. No single author has done as much to contribute to its continued confabulation.

I meant Tolkien may have originated the term "fell beast", not "fell" in general.

Interesting, I've been saying this for a long time without even knowing what it was. I think I said "in one fall swoop" TIL I guess.