Qualification of Absolutes
From Style in Guide part of ABC of Plain Words by Sir E Gowers (1951)

Certain adjectives and adverbs cannot properly be qualified by such words as more, less, very, rather. Unique is the outstanding example. When we say a thing is unique we mean that there is nothing else of its kind in existence; rather unique is meaningless. But we can of course say almost unique.

It is easy to slip into pedantry here, and to condemn the qualification of words which are perhaps strictly absolutes but are no longer so treated — true, for instance, and empty and full. We ought not to shrink from saying "very true", or "the hall was even emptier today than yesterday" or "this cupboard is fuller than that".