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Phrase(s): Jekyll and Hyde
someone with both an evil and a good personality. (From the novel The Strange Case of Dr. Jekyll and Mr. Hyde by Robert Louis Stevenson.) • Bill thinks Mary is so soft and gentle, but she can be very cruel—she is a real Jekyll and Hyde. • Jane doesn’t know that Fred is a Jekyll and Hyde. She sees him only when he is being kind and generous, but he can be very cruel.
McGrawhill's American Idioms And Phrasal Verbs
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A personality alternating between good and evil behavior, as in You never know whether Bob will be a Jekyll or a Hyde. This expression comes from Robert Louis Stevenson's The Strange Case of Dr. Jekyll and Mr. Hyde (1886). Also see LEAD A DOUBLE LIFE.
American Heritage Idioms