A washboard is a rectangular board with a ridged surface that people once used to scrub clothes clean before washing machines were invented. The board typically had metal or wooden ridges running across it. You'd wet the clothes, rub soap on them, then scrub them up and down against those bumps. The friction between the fabric and the ridges helped knock dirt loose. Doing laundry this way was hard, tiring work that could take hours, especially for large families.

People started using washboards in the early 1800s, and they remained common in American homes until washing machines became affordable in the mid-1900s. Some people still use them today when camping or in places without electricity.

The word also describes something that looks like a washboard's ridged surface. A bumpy dirt road with regular ruts might be called a washboard road because driving on it feels like rolling over those ridges. Musicians in folk and bluegrass bands sometimes use actual washboards as instruments, wearing metal thimbles on their fingers and scraping them across the ridges to create a rhythmic, scratchy sound. And when someone has extremely visible abdominal muscles, people jokingly call them washboard abs because the muscles create that same ridged appearance.