柠檬入口不觉酸薄荷入口不觉凉
[níng méng rù kŏu bù jué suān bó hé rù kŏu bù jué liáng]
When lemon comes into my mouth I don't feel sour, and mint doesn’t feel cold when tasted, describes experiencing things normally perceived as unpleasant yet without adverse reaction. Suggests growing indifferent or immune to hardships over time, similar to the English phrase 'tough as nails'.