grammaticality - Is 'uniquer' a word?

My spellcheck doesn't complain about 'uniquer'. Is it a valid word?

Since unique means "one of a kind", 'uniquer' has no valid definition, but that doesn't prevent it from being a valid dictionary word.

Answer

Spellcheckers are never to be absolutely trusted. When in doubt, the dictionary should always come in handy. Indeed, we colloquially modify "unique" all the time, as in quite unique, very unique, and really unique! This usage is rightly frowned upon by an overwhelming majority, considering the meaning of the word "unique":

being the only one of its kind; unlike anything else

• 
  

  particularly remarkable, special, or unusual

  
  


• 
  

  ( unique to) belonging or connected to (one particular person, group, or place)

  
  

New Oxford American Dictionary, 2nd Edition

The question of whether or not unique can be compared is a matter of debate among experts of the English language. Here's a note on usage from Merriam-Webster:

Many commentators have objected to the comparison or modification (as by somewhat or very) of unique, often asserting that a thing is either unique or it is not. Objections are based chiefly on the assumption that unique has but a single absolute sense, an assumption contradicted by information readily available in a dictionary... In modern use both comparison and modification are widespread and standard but are confined to the extended senses...

The "extended senses" would be the bulleted ones in my Oxford definition. And here is one more quote from Wiktionary:

Unique in its undisputed sense is not comparable. In its disputed senses, however, it can be compared as more unique, most unique, and occasionally as uniquer, uniquest.

Thus, uniquer can be considered valid within a proper context.