tough as nails 极为强硬（成语）

英文释义

Extremely stubborn, determined, and unyielding. (NOTE: often used in reference to someone’s negotiation style)

例句

We tried to buy a car for less than the original price, but the seller was as tough as nails, and he refused to give us any discount.

我们想以比原始价更低的价格买一辆车，可是销售人员极其强硬，拒绝提供任何折扣。