1.
a piece of cake, a cinch, easy task
Something so easy it can be done before breakfast. Used to describe tasks that require minimal effort.
Etymology
Literally "before breakfast" (朝飯前). The idea is that the task is so simple it can be finished even before eating the morning meal.
Usage
Most commonly used as a predicate with だ: 〜は朝飯前だ. Carries a confident, sometimes boastful tone. Tends to be used in informal speech.
Note that 飯 is an informal word for "meal/rice," which gives this expression a casual flavor.