Definition of Jekyll and Hyde noun from the Oxford Advanced American Dictionary

 

Jekyll and Hyde

 noun
noun
NAmE//ˌdʒɛkl ən ˈhaɪd//
 
[singular]
 
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a person who is sometimes very pleasant (Jekyll) and sometimes very unpleasant (Hyde) or who leads two very separate lives He leads an almost Jekyll and Hyde existence—by day he's an accountant, by night he's a singer in a rock band. From the story by Robert Louis Stevenson, Dr. Jekyll and Mr. Hyde, in which Dr. Jekyll takes a drug which separates the good and bad sides of his personality into two characters. All the negative aspects go into the character of Mr. Hyde.
See the Oxford Advanced Learner's Dictionary entry: Jekyll and Hyde