In "Keep it simple, stupid", is stupid addressing the reader or describing how to keep it?

Worth adding that the phrase circulates in several variants, and some of them do use the second structure — versions along the lines of "keep it short and simple" appear in print from the first half of the twentieth century.

So there is a family of similar advice, and the specific four-word form with the comma is one member of it. Some of the variants exist precisely because people found the insult version too rude to print, and softened it.

That is a useful reminder about phrases of this kind: they usually have several attested forms and a tidy single origin story is often constructed later. Accounts attributing it confidently to one person and one moment should be treated with some caution.

There is a spoken test that settles it, which is nice because it uses the intonation question from the other thread.

Direct address in English is marked prosodically — a slight break before the term of address, and it sits on its own contour. Say the phrase aloud naturally and you will find yourself doing exactly that: a small pause, then the last word set apart.

If it were a second adjective it would be integrated into the phrase with no break, the way "keep it short and simple" is.

So native speakers already parse it as address without thinking about it, and the punctuation is recording what the speech does.

It is the first reading: the comma marks direct address, and the phrase means "keep it simple, [you] stupid."

Grammatically the second reading does not work. A comma joining two adjectives after "keep it" would require them to be parallel modifiers of the same object, and the construction would need to be something like "keep it simple and stupid". Using a bare comma there is not standard English and would be ambiguous even if it were.

The address reading is also what makes the phrase memorable. It is a deliberately blunt piece of advice with the insult built in — a design principle that mocks the designer for wanting to be clever.

And the insult is not really aimed at anyone. It is aimed at the impulse to over-engineer, which is why engineers repeat it about their own work.

One small footnote: the comma is frequently dropped in modern use, which is what allows the ambiguity to arise at all.

Without it, the phrase does read as though it could be two adjectives, and a fair number of people have arrived at exactly your question that way. The abbreviated form, being letters only, drops the punctuation entirely and takes the question with it.