wash·board 
/ˈwɑ:ʃˌboɚd/ noun
plural wash·boards /ˈwɑ:ʃˌboɚd/ noun [count] 1 : a board with ridges on its surface that was used in the past for washing clothes by rubbing wet clothes against it ◊Washboards are also sometimes used as musical instruments in some types of folk and country music.
▪ The band featured a banjo and the rhythms of the washboard. 2 US, informal : a group of strong and well-shaped muscles that can be seen on a person's stomach — usually used before another noun ▪ a washboard stomach ▪ washboard abs
▪ The band featured a banjo and the rhythms of the washboard.
