hard as nails ------ 铁石心肠（成语）
英文释义
(IDIOM) Unyielding; lacking compassion; unsympathetic.
The wealthy old woman was as hard as nails, and never gave help of any kind to her friends or family members if they were experiencing difficulties.
这位有钱的老妇人是铁石心肠，在朋友或家人有困难时从不给他们任何形式的帮助。