[Dialect] a piece of cake, which means something really easy to do

The expression relates to an American word ‘cakewalk’, which was a popular competition in 19th century. In the cakewalk competition, couples walked arm-in-arm and the most graceful won a cake. So people use ‘it was a cakewalk’ to show that something was really easy, and later it developed into the expression ‘a piece of cake’.