The thinking is that unique properly only describes what is unequaled or otherwise distinct from all others. Just as something cannot be more "only" than another, it cannot be more unique than another.

UNIQUE definition: existing as the only one or as the sole example; single; solitary in type or characteristics. See examples of unique used in a sentence.

if something is unique to a place or situation, it only exists in that place or situation:

Definition of unique adjective in Oxford Advanced Learner's Dictionary. Meaning, pronunciation, picture, example sentences, grammar, usage notes, synonyms and more.

You can use unique to describe things that you admire because they are very unusual and special.

unique 1. used to mean 'the only one' If something is unique, it is the only thing of its kind. This is a unique opportunity. Humans are unique because they have the capacity to choose what they do.

If something is unique, it's the only one around. You might think getting a nose ring would make you unique, until you see five other people on the street with the same piercing. If someone calls you …