Extremely easy to do; requiring little effort or skill to accomplish.
This American phrase from the 1890s is somewhat ironic since making pie from scratch is actually quite difficult and requires considerable skill. The phrase likely refers to how easy pie is to eat rather than make, or it may have evolved from the earlier phrase "nice as pie," meaning pleasant or agreeable.
The irony of this phrase is delicious - anyone who has tried to make a proper pie crust knows it's one of the more challenging baking tasks! This disconnect between the phrase's meaning and reality makes it a perfect example of how idioms can evolve in unexpected ways.
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