At one fell swoop

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.

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.