"A wolf in sheep's clothing" is a dangerous person pretending to be harmless. Example: "I want you to stay away from that boy. He's a wolf in sheep's clothing." "A wolf in sheep's clothing" is a person who plans to do something bad ("a wolf") while pretending to be good and innocent ("in sheep's clothing"). Example: "I do not trust the salespeople at that store. They are all wolves in sheep's clothing." In past times wolves had a reputation as wild and dangerous animals that hunted and killed sheep. "A wolf in sheep's clothing" is a very dangerous wolf because we do not prepare ourselves for the attack. Example: "She may look cute, but that girl is a wolf in sheep's clothing."