The story
A metaphor referring to the fox as a predator that would prey on the chickens if given an opportunity to do so.
Etymology from Wiktionary, CC BY-SA 4.0.
fox in the henhouse
fox in the hen·houseToday it means: Someone untrustworthy placed in a position of trust in which they will be harmful, damaging or detrimental for their own personal gain. Full definition →