manger de la vache enragée

to endure a long period of hardship

It's about scraping by through genuinely tough times, often early in life, not about literally eating anything unwell.

Avant de réussir, il a mangé de la vache enragée pendant des années.

Before he made it, he went through hard times for years.

Goes back to at least the 17th century, from an age when the truly poor couldn't afford to be picky and would eat meat from a sick or 'rabid' cow that anyone better off would refuse.