Eren

There is a genuinely interesting edge case worth knowing, because it is the closest English gets: stress-based pairs.

A number of English words change part of speech depending on which syllable is stressed: the noun and the verb forms of words like record, permit, conflict and present. Change the stress and you have arguably a different word.

That is not tone, because stress involves loudness, length and vowel quality as well as pitch, and because it is a small closed set rather than a system. Linguists sometimes call English a stress-accent language for this reason.

But it does show that the boundary is a spectrum rather than a wall. Some languages use pitch accent - a system sitting between the two: where pitch on a particular syllable distinguishes words, without a full tone on every syllable.

There are several, and the difficulty you hit is that they each carry different baggage. The main candidates:

Principled, the most direct fit. Acting on principle, in the plain sense. "High-principled" intensifies it and is well attested.

Scrupulous, careful to do nothing morally wrong. Precise and slightly cold; it emphasises care and attention rather than conviction.

Upright - firm and honourable. Slightly old-fashioned and warm.

Conscientious, guided by conscience. Has drifted towards meaning "diligent at work", which weakens it for your purpose.

Righteous, technically correct and largely unusable, because in modern use it carries a strong implication of self-satisfaction.

For a straightforward description with no unwanted flavour, principled is the answer, and high-principled if you want the intensity.

One drafting note, since you rewrote the sentence: in practice the plain construction often reads better than any single word.

"A man of principle" is idiomatic, unambiguous, and carries no baggage at all. English frequently has the noun phrase where it lacks the adjective, and reaching for it is not a failure to find the word.

Your instinct to rewrite may have produced better prose than the word you were hunting for would have.

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.

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.

The historical wrinkle worth knowing: English used to have grammatical gender, and lost it.

Old English had three genders with full agreement, in much the way German does. The system eroded over several centuries, driven substantially by the loss of unstressed endings: once the endings that carried the distinctions wore away, the distinctions had nowhere to live.

A few fossils remain. The pairs you mentioned are one kind, mostly borrowed rather than native. The habit of calling a ship she is another.

So English is not a language that never had it. It is a language that had it and shed it, which is a fairly common trajectory and a reminder that these systems are historical accidents rather than features anybody chose.