Nouns and adjectives have slopped over each other since before the beginning. Even classical Latin did it.

Yes, indeed. That's why Latin grammarians called both nouns and adjectives nomina 'names'. As far as they, and the Greek grammarians whom they aped were concerned, nouns and adjectives were the same kinds of words. They did subdivide the grammatical category into nomina substantiva and nomina adjectiva. (Ever notice how entries for nouns and adjectives in the OED are categorized by sb. and a.?)

Ceci n'est pas un seing.