early 17th century: from French, from Latin unicus, from unus 'one'
There is a set of adjectives—including unique, complete, equal, and perfect—whose core meaning embraces a mathematically absolute concept and which therefore, according to a traditional argument, cannot be modified by adverbs such as really, quite, or very. For example, since the core meaning of unique (from Latin ‘one’) is ‘being only one of its kind,’ it is logically impossible, the argument goes, to submodify it: it either is ‘unique’ or it is not, and there are no stages in between. In practice, the situation in the language is more complex than this. Words like unique have a core sense, but they often also have a secondary, less precise (nonabsolute) sense of ‘very remarkable or unusual,’ as in a really unique opportunity. It is advisable, however, to use unique in this sense sparingly and not to modify it with very, quite, really, etc.