"mot juste" is a set phrase (from French) meaning a word that is exactly right for a particular situation. "mot" = "word" and "juste" = "right/appropriate", so "It is hard for a mot to get more juste than that" means "It is hard for a word to get more right/appropriate than that". The author is playing with words by breaking up the set phrase and using the French words for their individual meanings in a way that would not normally be done in English.
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