What does "wolf in sheep's clothing" mean?

An enemy disguised as a friend, as in Dan was a wolf in sheep's clothing, pretending to help but all the while spying for our competitors. This term comes from the ancient fable about a wolf that dresses up in the skin of a sheep and sneaks up on a flock. This fable has given rise to a rich history of allusions as in the Sermon on the Mount when Jesus speaks of false prophets in sheep's clothing, “but inwardly they are ravening wolves” (Matthew 7:15).