adjectival

First recorded in 1790–1800; adjective + -al¹

1350–1400; Middle English < Late Latin adjectīvum, neuter of adjectīvus, equivalent to adject(us) attached, added, past participle of ad(j)icere ( ad- ad- + -jec-, combining form of jac- throw + -tus past participle suffix) + -īvus -ive

How do we spot an adjective? For one thing, adjectives tell us about the nouns they qualify by answering questions like “what kind,” “which one,” and “how many”: a serious student; the purple flower; three kisses. But in English there are adjectives and there are adjectives. Those in the second group are more adjectival than the others, in that the qualifications they express can themselves be qualified. The word more is our clue; true adjectives can compare one entity to another. For adjectives with two or more syllables, the comparative and superlative are formed with more and most ( more captivating; the most enthralling ). Monosyllables, and some disyllables that happen to end in -y, change form, with occasional accommodations in spelling, by adding -er and -est: smart, smarter, smartest; happy, happier, happiest. There are, of course, irregular members of this group; despite what your average three-year-old says, things go from good to better and best, not to gooder and goodest. But there is a caution; some adjectives have absolute meanings that can make them seem absurd if used comparatively. If a plant is dead, for example, another plant cannot be more dead.
In addition, many true adjectives are gradable. That is, they can be upgraded ( very pretty ), downgraded ( somewhat disorganized ), or intensified ( really tired ). Usually, those that should not be compared, as correct, impossible, and mortal, are also not gradable. A vote, for example, cannot be very unanimous, too unanimous, or not unanimous enough; it is either unanimous or not. And only in The Wizard of Oz is the Wicked Witch “not only merely dead, she's really most sincerely dead.” That is not to say that there are no exceptions, as can be seen at the expanded usage note for the absolute adjective unique.
Pronouns, as your, this, and each, can also function as adjectives. But it is the noun as modifier, like bottle and bus in bottle cap and bus station, that gives headaches to dictionary compilers. Faced with evidence, they must ask themselves if occasional use as a modifier makes a particular noun worthy of full adjective status. Bottle and bus certainly do not pass comparison or gradation tests; my cap isn’t more bottle than yours, nor is it very bottle. These nouns are not listed as adjectives in this dictionary. Yet similar nouns, like coffee, kitchen, and summer, are. The number of items they can modify, the number of adjectival senses they have, and the degree to which such senses differ from their noun senses all play a part in the decision. That decision, however is never final. Meanings expand and evolve. Language changes as we speak.