pour des prunes
“Pour des prunes” is a French expression that literally means “for plums.” Its closest English equivalent is “for nothing.” The saying dates from the start of the 16th century.
However, the association of plums with a complete lack of value dates back to the end of the Second Crusade, which finished around 1150 AD and was generally a failure. The Crusaders brought back some plum trees from Damascus and showed them to the King, who was angered that the Crusaders had achieved nothing more of value than acquiring a few plum trees, which were something they could have had easily in Europe, without having to go on a Crusade to the Holy Land. Since then, the plum has been associated with something of little or no value, however delicious they might be.