While defining a given adjective as unique is subjective, writer and editor Mark Nichol offers adjectives such as concomitant, gustatory, mordant, platitudinous and friable as examples. Commonly used adjectives can also appear unique when used in unusual contexts.
Concomitant means accompanying. Gustatory refers to something that has to do with taste or eating.
Mordant describes a something that is pungent or incisive. Something that is platitudinous is full of dull comments. Friable means brittle.
Other unique adjectives listed by Nichol include contumacious, which means rebellious, and lachrymose, which means tearful. Something that is redolent is aromatic or evocative, while a querulous person is cranky or obnoxious. Sagacious individuals are wise, verdant settings are green and unspoiled, and when something is turgid, it is swollen.
About a quarter of words in English are adjectives or words that are sometimes used as adjectives. Estimates of the total words in English range from 250,000 to over a million, meaning somewhere between 62,000 and 250,000 adjectives likely exist. Most of these words do not find use in common language and tens of thousands of words are obsolete, meaning a majority of adjectives are unique compared to the most commonly used adjectives, such as good, new and first.