The story
Mondegreen variant of in one fell swoop, where "fell" (of a strong and cruel nature; eager and unsparing; grim; fierce; ruthless; savage) is misheard as "foul" (disgusting) or "fowl" (a bird).
Etymology from Wiktionary, CC BY-SA 4.0.
in one foul swoop
Today it means: (Achieved or completed) with a single action; in a single stroke. Full definition →