i.e. is the comma to signify the boundary between the instruction and the recipient of it (that is to say, "Hey, stupid! Keep it simple"), or is it serving the function of an "and" (as is sometimes used in US English but rare in British).
One thing to note is that the Wikipedia article on the subject is a bit specious:
First seen partly in American English by at least 1938 . . . the variant “Keep it Short and Simple” is attested from a 1938 issue of the Minneapolis Star.
Partly? Very partly; there is no sign of an acronym at play — no discernible throughline to KISS, no sign of “variation.”
A quick search of Google Books does indeed turn up keep it simple, stupid under the item Constitutionality of the General Accounting Office (1938), but one need only scroll into the document about 20 pages to see that it actually appears in a 1996 hearing of the Committee on Government Reform and Oversight.
I can find the phrase in print as early as 1961, in a Popular Mechanics column titled “Sidelights from the Pentagon”:
PROJECT KISS is a current Navy program which stands for, not the old social pastime, but for a new slogan, “Keep It Simple, Stupid!” It’s a reminder that simple design of weapons systems increases reliability and cuts costs.
By the early 60s, it seems the term was “old” — in common usage. This is from Five Years to Freedom — The True Story of a Vietnam POW (originally published in 1971), wherein the author recounts a 1964 experience of his:
The old “KISS” formula, “Keep it simple, stupid,” served as my guide as I built the biography which would allow me to deny knowledge of the areas they have displayed an interest in.
With or without a comma, none of this particularly proves whether stupid was meant as a form of direct address or not (I doubt that generals and engineers think much about the subtleties of punctuation).
But if simple is good here, then simple is smart — not stupid. That would leave — you (the addressee) as the primary candidate . . .
It's rare to use a comma between a pair of predicative adjectives unless one is in apposition (a synonym to elucidate [a] or a near-synonym to  develop,  amend (make a minor correction or add precision) or  bracket (where an 'and' is considered too divisive and a slash too informal) the required sense [b]):
[a] [T]he revelations have been uniformly anticlimactic. The exchanges, most reports agree, are quotidian, mundane. [Rebecca Traister; The Cut; 2015]
[b1] [The dog was] found to be very thin, emaciated .... RSPCA Cymru; Facebook; 2018]
[b2] [W]hile the reserves are large, enormous, at the present time, the battalions have been far under strength. [Hansard; Army Estimates; Vol 160; 1906-7]
[b3] She claimed that Lord Haw Haw's words were misleading, disingenuous.
[b3] The results of the experiment were disappointing, frustrating.
Since this is not the case with 'keep it simple, stupid', it is highly likely that 'stupid' here is a vocative, as in 'keep it simple, Stan'.
Like many idioms, different interpretations are often just as valid as one another.
"Hey Stupid, Keep it simple!" is pretty self explanatory. "Don't be stupid, do things simply"
"Keep it simple (and) Stupid" works as an admonishment to keep your work straightforward and choose the "dumb" approach rather than being too clever about it.
Anecdotally, I work in the software industry, and there are always many many different ways to accomplish any given task.
In general, the accepted attitude in my place of work is that it's better to write a big sprawling piece of code using simple techniques than a clever and efficient solution.
Clever code is dense code, and dense code is often difficult to immediately understand for other developers.
Chunky and Dumb code is comparatively easy to pick up and modify without wasting serious time analysing what your clever colleague did.
Given that time is money and there is no prize for writing beautiful code, it's almost always better to reserve your cleverness for things that really merit it.
"Keep it simple and stupid" is a mantra in my team.
As a native english speaker, it clearly reads to me "keep it simple [you] stupid [person]". The tone is lighthearted although slightly brash.
To avoid the brashness, there are alternate variations (which are most certainly backronyms):
- "keep it simple, silly"
- "keep it stupid simple" (which means "keep it utterly simple")
But to underscore: the usage of the term "stupid" squarely describes a proverbial person.
Some suggest that the phrase has a softer meaning, but this is not at all true. Nobody would phrase a statement like "keep it simple utterly" unless rambling when speaking. And consider that the word "stupid" here is a shortened form of the word "stupidly", having its
-ly suffix omitted. In every case where "stupid" is used as an adjective without its suffix (e.g. "stupid simple", "stupid easy", "stupid smart") it always precedes its object. It never comes after. So the plain interpretation (and only interpretation) is "keep it simple, [you] stupid [person]".
Having been a techie for the last 50 years, on and off, here is what I know. It is really easy when on the hunt for the "perfect solution" to go down a rabbit hole of modifications to the initial design, with a result that is less than robust (think Rube Goldberg).
"Keep it simple, Stupid" is a rejoinder the designers/engineers gave to THEMSELVES to keep focused on the solving the problem and not keep trying to fix the solution.
Calling yourself Stupid should not offend anyone.
I’ve always understood it as a vocative (Keep it simple, [O] stupid [person].) There are other similar sayings, like James Carville saying about the 1992 U.S. presidential election, “It’s the economy, stupid.”
I’d say the other way, “Keep it Simple and Stupid,” or “Keep it stupidly simple.” I don’t think a list without a conjunction would be grammatical here.
I’ve also frequently heard, “Keep it simple, shithead.” (No, not addressed at me.)