A wolf in sheep's clothing is an idiom of Biblical origin used to describe those playing a role contrary to their real character with whom contact is dangerous, particularly false teachers.
The phrase comes from the Sermon on the Mount, in the Gospel of Matthew 7:15: "Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravening wolves." The warning is paired in the same passage with its own diagnostic — the disguised are known by their fruits — which is why the idiom carries an implied instruction to judge conduct rather than presentation.
It is routinely credited to Aesop, but no fable on this precise theme survives in ancient Greek sources. The earliest documented version was written by the twelfth-century Byzantine rhetorician Nikephoros Basilakis in his Progymnasmata, and the telling behind most modern retellings comes from the fifteenth-century Italian writer Laurentius Abstemius. The Perry Index catalogues three related Greek fables — 234, 267, and 451 — in which shepherds wrongly trust a wolf, none of which involves a disguise.
Much later, the idiom has been applied by zoologists to varying kinds of predatory behaviour, specifically aggressive mimicry: a predator that resembles something harmless or that impersonates another species in order to approach its prey, documented in jumping spiders, lacewings, cichlid fish, and the zone-tailed hawk. Aggressive mimicry is the offensive inverse of Batesian mimicry, in which a harmless species advertises itself as dangerous. Applied to people, the idiom names the same structure that 📝Masking describes clinically — a persona convincing enough to secure access, deployed by someone whose intent the persona conceals — and it remains a general-purpose label for 📝manipulation discovered after the fact.
