be a piece of cake

To be a very easy task or accomplishment. I thought I was going to fail the test, but it turned out to be a piece of cake!

It is laughably simple; it’s easily accomplished. This term is supposedly derived from the cakewalk, originally (mid-nineteenth century) an African-American promenading contest in which couples who devised the most intricate or appealing steps won a cake as a prize. Later the phrase came to mean a high-stepping dance with a strutting step, based on the promenade, as well as the music for such dancing. Finally, by the early twentieth century, cakewalk came to be slang for something stylish, pleasurable, and easy to do, and by the late 1930s it had been converted to piece of cake. Both piece of cake and cakewalk were British armed forces slang for an easy mission during World War II, and the former was used as the title for a television drama (1989; 1990 in America) about the Royal Air Force during that conflict. See also easy as pie; duck soup.