Mal du siècle refers to a feeling of sadness, despair, and meaninglessness that was commonly experienced by people during the 19th century. People thought that society was changing too rapidly, and traditional ways of life were being destroyed. Many people felt lost and unsure about their place in the world. It was like feeling homesick, but not for a place, rather for a time where things were simpler and happier. It was a sense of longing for a time that was gone forever. Today, the term mal du siècle is used to describe similar feelings of unease and dissatisfaction with the modern world.