It comes from a 1970's british TV commercial for Lemon Squeezy detergent. They were with a little girl who points out dirty greasy dishes to an adult (mom or relative) and then this adult produces Lemon Squeezy and they clean the dishes quickly. At the end of the commercial the girl says "Easy Peasy Lemon Squeezy".

Today it is a silly way to state something was or will be very easy.

Lying flat refers to a defeatist lifestyle in Chin, where people stop working, desiring material acquisition, and tap out of any social life – sometimes for good. By lying flat, young people are participating in a kind of non-cooperation movement, rather than surrendering themselves to China's oppressive work culture where they see little hope of social mobility.

(uk) very easy (short for easy-peasy-lemon-squeezy)

the first half of a rhyming phrase with several alternate second halves, all of which connote an activity or a result that is, respectively, simple to perform or achieve.

1. The first half of a rhyming phrase with several alternate second halves, all of which connote an activity or a result that is, respectively, simple to perform or achieve: Very Easy

2. An operating system based on Ubuntu for netbooks: www.geteasypeasy.