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Phrase(s): at one fell swoop [and] in one fell swoop
Fig. in a single incident; as a single event. (This phrase preserves the old word fell, meaning "terrible" or "deadly.") • The party guests ate up all the snacks at one fell swoop. • When the stock market crashed, many large fortunes were wiped out in one fell swoop.
McGrawhill's American Idioms And Phrasal Verbs