From Chinese folklore:
狐狸精 (fox spirits) can shapeshift into beautiful humans, but their tails are the one feature they can never fully hide. So
露出狐狸尾巴 ('to show one's fox tail') means a cunning or deceitful person has finally given themselves away. Almost always used in the verb pattern
露出狐狸尾巴 — the giveaway, not the disguise.
