An adjective was first an adjective, not a noun.
An adjective is something of a turncoat. When borrowed from French around the early 1400s, adjective was the adjective in the phrase “noun adjective” (for a clearer grasp on this, stress the second syllable, as you would the word objective). It was apparently so impressed with its companion noun noun that it came to the dark side, Luke, and now works almost exclusively as a noun.