school of hard knocks

noun

1. the experience gained from living, especially from disappointment and hard work, regarded as a means of education: The only school he ever attended was the school of hard knocks.

The practical experience of life, including hardship and disappointments. For example, A self-made man, he never went to college but came up through the school of hard knocks. This idiom uses knock, “a blow,” as a metaphor for a setback. [Mid-1800s]