a wolf in sheep's clothing

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