The phrase "as easy as ABC" is an idiomatic expression used to convey that something is very simple or straightforward.
While it's challenging to pinpoint the exact origin of such common idioms, the expression "as easy as ABC" has been in use for a long time and likely evolved organically over the years.
The use of alphabetical order as a metaphor for simplicity has ancient roots, but the specific phrase "as easy as ABC" might not have a single, well-documented origin. It's probable that variations of this expression have been used in spoken language for centuries.
The phrase itself is a simple metaphor, comparing the ease of a task to the simplicity of the alphabet, where ABC refers to the first three letters. The idea is that just as learning or understanding the alphabet is basic and straightforward, so is the task or concept being described.