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An enemy disguised as a friend, as in Dan was a wolf in sheep's clothing, pretending to help but all the while spying for our competitors. This term comes from the ancient fable about a wolf that dresses up in the skin of a sheep and sneaks up on a flock. This fable has given rise to a rich history of allusions as in the Sermon on the Mount when Jesus speaks of false prophets in sheep's clothing, "but inwardly they are ravening wolves" (Matthew 7:15).
American Heritage Idioms
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Phrase(s): a wolf in sheep’s clothing
Fig. a dangerous person pretending to be harmless. • Carla thought the handsome stranger was gentle and kind, but Susan suspected he was a wolf in sheep’s clothing. • Mimi: Why shouldn’t I go out with David? He’s the nicest man I’ve ever met. Alan: He’s a wolf in sheep’s clothing, Mimi. Can’t you tell?
McGrawhill's American Idioms And Phrasal Verbs