What part of speech is mimsy?

The Word 'mimsy' is a Word invented by author Lewis Carroll first appearing in his poem "Jabberwocky". Although it has been used by other authors since, there is no clear idea exactly what a 'mimsy' may be. Carroll used the Word as a common noun "All mimsy were...", subject of the sentence.

Example sentence: It is up to a writer's imagination what a mimsy is.