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A task that's easy to accomplish; something that's lacking complexity.

Have you ever heard someone say: "It's not exactly brain surgery"? Or perhaps
you've heard the phrase "it's not rocket science." Why do people say such
things? Well, it's believed that professions like brain surgeons, rocket scientists,
neurologists, among other complicated jobs, require an intelligent mind to succeed at. Hence, when a person uses these phrases, they are implying that what you're doing is rather simple in comparison to these other, more complex professions.

So when did this saying orginate? It took a bit of time for brain surgery to become a successful procedure. For example, the first operation to treat a brain tumor happened in 1884. While the patient survived the operation at first, he died about a month later from meningitis, so it wasn't a very successful surgery. It took until the early 20th century for brain surgery to become a more frequent and successful treatment, so it's likely that during this century is when the phrase came into existance.

Interestingly enough, this figure of speech looks to be relatively recent, only going back as far back as the 1970s from what I've found. It's used in the Ames Daily Tribune from 1971 in an article about women repairmen becoming more common:

"He determined that his book, which is as yet untitled and will be published by Prentice-Hall late next
year, would dispel the mystery and get women over the 'original hurdle--the psycological block.' 'After
all,' he added, 'it's not like brain surgery.'"
Note: The origins for most phrases and sayings cannot be said with a certainty. What's provided are theories that may be plausible to how a phrase originated, but not necessarily so.

In addition, quotes that contain a particular phrase may be taken from old newspapers, poems, or books that were written centuries ago, but this by no means confirms that the phrase originates from said newspapers, poems, or books. In all likelihood, if an expression is being used in a newspaper, it's probably already a well known saying and is from an older time.
* My wife was trying to put together a bike for our child, but she could not figure out what she had to do next, so I told her it wasn't brain surgery and we finished it together.
Know Your Phrase - The Origins And Meanings For Idioms And Phrases > I-Letter Sayings
The meaning of this phrase.
The origins of this idiom.
Phrases and Idioms - Meanings and Origins of Idioms | If you want, feel to look at some dog, cat and animal phrases.