The French saying “travailler pour le roi de Prusse” translates literally as “to work for the King of Prussia.” What it really means is “to work for nothing,” or “to not be paid for your efforts.”

The idiom first appeared around the middle of the 19th century. Its origin remains unclear, however. There are a few different possible explanations, none of which agree.

The first explanation suggests that the salaries paid to Prussian mercenaries at the start of the 18th century were very low.

The second explanation links the expression to a 1757 song which made fun of the Prince de Soubise at Rossbach. This song contained the phrase “he worked for the King of Prussia.”

The third possible explanation says that the expression comes from Frederick William I, who was the father of Frederick II. Frederick I was cruel and greedy. He was renowned for underpaying people.

The nearest equivalent English expression is “to work for peanuts.”