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Adjective: Describes a Jeckyll and Hyde like character - a person who is apt to change from a good person to an evil person (combination of benevolent and malevolent).
Dr Jeckyll is manevolent, as he shifts quickly from caring doctor to evil and violent murderer Mr Hyde.
Mary-Lou can be lovely and kind one minute and spitefully vindictive the next - she is such a manevolent cow.

'Jack the manevolent nurse was happy by day caring for his helpless patients while all the while plotting how he could overdose them with morphine when the lights went out'
by LouLou333 April 05, 2010