mischief + -ous

source: A Mischievous Smile | Anime Amino

(of a person, animal, or their behavior) causing or showing a fondness for causing trouble in a playful way:

〈人が〉いたずら好きな; 〈笑み･表情などが〉いたずらっぽい

e.g. two mischievous kittens had decorated the bed with shredded newspaper.

(of an action or thing) causing or intended to cause harm or trouble:

｟かたく｠ 〈行為･言葉などが〉(故意に)問題を引き起こす; 人を傷つける, 害を与える.

e.g. a mischievous allegation for which there is not a shred of evidence.

mischievousness |ˈmisCHivəsnəs| noun

Middle English: from Anglo-Norman French meschevous, from Old French meschever ‘come to an unfortunate end’ (see mischief). The early sense was ‘unfortunate or calamitous’, later ‘having harmful effects’; the sense ‘playfully troublesome’ dates from the late 17th century.

Mischievous is a three-syllable word. Take care not to use this incorrect four-syllable pronunciation: |misˈCHēvēəs|