The story
From Matthew 7:15 in the Bible (King James Version; spelling modernized): “Beware of false prophets which come to you in sheep’s clothing, but inwardly they are ravening wolves.”
Etymology from Wiktionary, CC BY-SA 4.0.
wolf in sheep's clothing
wolf in sheep's cloth·ingToday it means: Someone who or something which is harmful or threatening but disguised as something peaceful or pleasant. Full definition →