Liliputin -1886

Sepukku is not something easy peasy Japanesey ... "
Madame Butterfly

Liliputins. What, the heck, is this?
http://stihi.ru/2021/11/24/7101

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What does “Easy peasy japanesey” mean?

Especially "japanesey", why Japan?

It was in the 1994 movie The Shawshank Redemption.

phrases meaning-in-context

It means it's very easy. Japanesey, I would guess, for no better reason than that, like peasy, it rhymes with easy.

additional info from Magy:

James Whitmore, who played Brooks Hatlen character (and who said "easy peasy, japanesey"), was very well known actor. Digging a bit deeper in history of this actor revealed his serving with the Marines in World War II and such experience he employed in numerous movies from that period and storylines: as Vice Adm. William F. 'Bull' Halsey Jr. in famous Tora! Tora! Tora!, then in Nobody's Perfect, The Deep Six(1958), Above and Beyond(1952) and others. It's highly likely, this speech was Whitmore own contribution into movie, to give more direct and realistic feel of his character. There is also possible, that's phrase/slang was used amongs Navy crews during WWII.

It is easy peasy, Lemon Squeezy in my part of the world, after a long-defunct brand of washing-up liquid. –
Then, of course, there's the simple guide to computing, 'Easy PC'.
Wiktionary states it is a nonsensical rhyme – Unreason Nov 8 '11 at 10:41

Or, as the Canadians would say, Eee Zed Pee Zed.