A wolf in sheep's clothing is an idiom from Jesus's Sermon on the Mount as narrated in the Gospel of Matthew. It warns against individuals who play a duplicitous role. The gospel regards such individuals (particularly false teachers) as dangerous.
Fables based on the idiom, dated no earlier than the 12th century CE, have been falsely credited to ancient Greek storyteller Aesop (620–564 BCE). The confusion arises from the similarity of themes in Aesop's Fables concerning wolves that are mistakenly trusted, with the moral that human nature eventually shows through any disguise.
In the modern era, zoologists have applied the idiom to various types of predatory behaviour.