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Note: The phrase origins, or history of many common idioms can be impossible to trace most of the time. Generally, what's provided are the plausible theories floating around for each saying.

Quotes will often be taken from old books, newspapers, poems, plays, etc, as an indicator for readers to see how far back in history these phrases go. However, keep in mind that if a newspaper from the 1850s is using a certain idiom, then that idiom is probably already well-known during those times, so it's likely much older.
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Common Phrase Origins and Meanings > E-Letter Idioms
The meaning of the phrase as Easy as Pie.
Origins for the idiom Easy as Pie.
This phrase is believed to come from the pleasantness and ease involved when eating a delicious pie. Basically, something is as easy as eating a pie. Evidently, during the 19th century, the word 'pie' was used to describe someone as being delightful or to depict something as being easy. Consider the examples below:

In the year 1885, the word 'pie' was used as a nice remark by Mark Twain in The Adventures of Huckleberry Finn, where it reads:

"You're always as polite as pie to them."

The popular saying we're familar with today is found just two years later in 1887. It's written in various newspapers from that year, one of them being a newspaper from Rhode Island called Newport Mercury, and it says:

"You see veuever I goes I takes away mit me a silverspoon or a knife or somethings, an' I gets two
or three dollars for them. It's easy as pie. Vy don't you try it?"

So the phrase dates back at least to the late 19th century.
A task or job that is simple or pleasurable to finish, requiring little effort; simple.

Peoples' lives are filled with things to do. Some things are easier to do than
others. When someone considers an activity to be easy, they may use idioms
and similar phrases like this one to describe the simplicity involved.
* Having such a busy schedule today prevents me from making a hearty breakfast for myself, so I'll just make a quick bowl of cereal for myself instead since it is as easy as pie to make.

* Janice didn't know how to swim and was afraid to be around water, however, after a few swimming lessons, she became more comfortable in the water and found swimming to be easy, like pie.
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