In the Middle Ages, priests and monks sang in their church choirs just as people do today. Sometimes they had to sing for hours, standing up, and their legs would get very tired. But they weren't supposed to sit down.
Misericorde from Walcourt, Belgium (1500s AD)
So the monks built folding wooden seats into their church choirs, where the choir stood (It's called the choir because that's where the choir stands). These seats had to be folded up when the choir was supposed to be standing up, but they had little shelves built onto them so that you could lean on them and still look like you were standing up. People called these seats with the shelves "misericordes," (MEE-ser-i-CORDS) because that means "pity" in French, and these were seats that took pity on your tired legs.
While they were at it, the people carving these wooden seats often carved funny sculptures into the underside of the seats, which you could see when the seats were folded up. Sometimes they're cute bunnies or foxes. Other times they show people being bad, or just being funny. One misericorde shows people going to the bathroom!