early 17th century: from French, from Latin unicus, from unus 'one'
There is a set of adjectives—including unique, complete, equal, infinite, 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 in-between stages. 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 sense: in this case, the meaning ‘very remarkable or unusual’, as in a really unique opportunity. In its secondary sense, unique does not relate to an absolute concept, and so the use of submodifying adverbs is grammatically acceptable.