(exp)a cornered rat will bite a cat; despair turns cowards courageous
「そんなに舐められた相手じゃないけどなぁ・・・」「そうだよ、窮鼠猫を噛むって言うじゃないか」 "I don't think she's an opponent you can ignore like that ..." "That's right, they say a cornered rat bites back, don't they?"
I wouldn't push him too far. You don't know what he might do. I'd say you can't be too careful. They say even a doomed mouse will bite a cat if he has no choice.
Japanese Dictionary
