Which is Correct: One foul or fowl swoop? (Neither: It's one fell swoop.)

The other day I tried to use a cliche phrase one foul swoop.

Once I wrote it, I wondered about the original meaning. Was there a foul, like a mistake swooping in?

Or maybe the phrase refers to a fowl bird that swoops down all at once.

It turns out that neither is correct. Instead the phrase should be one fell swoop.

One fell swoop means all at once. It appears in Shakespeares play, Macbeth.

While we dont often use the word fell in this way anymore, it meant fierce, villain, terrible,...