This idiom describes someone or something that appears harmless or benevolent but is actually dangerous, deceitful, or malicious. It warns against trusting appearances, as the ‘wolf’ hides its true nature under a ‘sheep’s’ guise. The phrase is used to caution about hidden threats in people, ideas, or situations.

The phrase originates from Aesop’s fables (6th century BCE), specifically ‘The Wolf in Sheep’s Clothing,’ where a wolf disguises itself to prey on sheep. It appears in the Bible, Matthew 7:15 (King James Version, 1611): ‘Beware of false prophets, which come to you in sheep’s clothing, but inwardly they are ravening wolves.’ By the Middle Ages, it was a common proverb in Europe, with Chaucer using it in *The Canterbury Tales* (1387). Its use in English literature, including by Shakespeare and Milton, solidified its warning against deception.