A piece of cake is something extremely easy to do. It was a piece of cake, a piece of cake for him, hardly a piece of cake.
It is always a boast made afterwards, since nobody calls a thing a piece of cake before attempting it. The phrase is American and dates from the 1930s, and the Royal Air Force took it up in the war.
1phrasepiece of cakeSomething extremely easy to do.
“Don't worry about the test; it'll be a piece of cake.”
piece of cake is an ease.
First attested in American English in 1936, perhaps from the cakewalk or simply from cake as an easy prize.
Attested since 1936, originally in American English. Possibly from cakewalk, or the notion of facility that derives from many cakes having agreeable tastes, and hence being ‘easy’ to consume. Compare French être du gâteau literally “to be a cake”, être de la tarte literally “to be a pie”, both meaning "to be easy, to be a piece of cake".
Etymology from Wiktionary, CC BY-SA
Practised in Idioms