王八吃秤砣 ——— 铁了心
A tortoise swallowing a weight — get an iron heart
王八 is an alternative vulgar name for a tortoise. 秤砣 is the iron weight sliding along the arm of a steelyard. If a tortoise swallowed a weight, he would feel as though he had got an iron heart. 铁了心 means having made up one's mind or having an iron will.