a wolf in sheep's clothing
US /ə wʊlf ɪn ʃiːps ˈkloʊðɪŋ/
UK /ə wʊlf ɪn ʃiːps ˈkloʊðɪŋ/

1.
羊の皮をかぶった狼
a person who appears friendly or harmless but is actually dangerous or malicious
:
•
Be careful of him; he's a wolf in sheep's clothing.
彼には気をつけろ。彼は羊の皮をかぶった狼だ。
•
The new manager seemed nice at first, but he turned out to be a wolf in sheep's clothing.
新しいマネージャーは最初は親切そうに見えたが、実は羊の皮をかぶった狼だった。