malicious; scheming; black-hearted
Describes a person who appears kind or harmless but harbors ill intent or cunning plans.
He's a malicious guy.
Behind that smile lies a scheming mind.
Manga slang for someone whose cruelty is hidden behind a friendly facade.
That character is famous for being sly and cruel.
Compound of 腹 (belly, mind) and 黒 (black), literally 'black-hearted'. The exact historical derivation is uncertain, but the meaning is transparent from the kanji.