More than any other word or phrase covered in my book Gobsmacked!: The British Invasion of American English—more than “kerfuffle,” more than “go missing,” more than “spot-on,” more than “gobsmacked” itself—the origin of “easy peasy” is shrouded in mystery, or, at least, uncertainty.
For starters, most British people’s belief that it came from an advertising slogan for Sqezy dishwashing liquid, “Easy Peasy, Lemon Sqezy,” is Mandela-effect-level flat-out wrong. The language historian Barry Popik and Pascal Tréguer, who runs the Word Histories blog, have both established that there was never such a slogan. Rather, from 1957 until 1962, the product’s tagline was “It’s Easy with Sqezy.” Shortly after that, the brand was discontinued. It wasn’t until two decades later that the first instance of the full phrase was spotted (by Tréguer), in a 1983 article in The Guardian: “Chap comes in, sits down, says, ‘I want to be a marine biologist.’ Easy peezy lemon squeezy.”