fell 4adjective poetic/literaryof terrible evil or ferocity; deadly : sorcerers use spells to achieve their fell ends.PHRASESin (or at) one fell swoop all at one time : nothing can topple the government in one fell swoop. [ORIGIN: from Shakespeare's Macbeth ( iv. iii. 219).]
I have a confession to make: I've not till recently knew the many meanings of fell, aside from past tense of fall. I simple accepted one fell swoop as is, possibly because the expression is so wonderfully sinuous it must be right. Like a snake plunging in for the kill.