“Fell” is also a noun, a verb, and the past tense of the verb “to fall”, but that is not what gets mixed up here.
To do something in one fell swoop means to do it all at once. One action can accomplish an entire task. The noun swoop dates from the 1540s, and represents the action that a bird takes when pouncing on its prey. One fell swoop—one deadly pounce—meant that the prey was caught in one shot. No need for further attempts.