Washboard muscles, also known as abdominal muscles, are a group of muscles located in the abdomen. They are responsible for providing stability and support to the spine and trunk, as well as aiding in breathing and posture. The term 'washboard muscles' is derived from the appearance of the abdominal muscles when they are well-defined and toned. ABS is an acronym for the rectus abdominis muscle, which is the most prominent of the abdominal muscles and is responsible for the 'six-pack' look.