The phrase “piece of cake” comes from the idea of something being simple or easy to accomplish, much like eating a sweet, pleasant treat like cake. It originated in the early 20th century and became popularized due to its use in British and American English. Some sources trace its roots to competitions where cakes were given as prizes, emphasizing that the task to win was as simple as receiving a “piece of cake.” It is believed to have become widely known through the influence of American culture, including media, during the 20th century.