狐假虎威

A tiger caught a fox in a forest, and was just about to eat it, when the fox said, 'You mustn't eat me. I was sent by Heaven to rule the animals. By eating me, you will violate（违反，侵犯） the command of Heaven. If you don't believe me, just follow me to see whether the animals are of me.' The tiger agreed, and followed the fox as it walked around the forest. The animals all ran away on seeing them. The tiger thought they were of the fox, so he let it go. He didn't realise that it was him that the beasts were really of.

This idiom means relying on another's power to bully（欺负） or frighten others.