Heck no, this is a term I learned from CJ and Marius. "Educated" people speak with very precisely correct (higher register) English. "Uneducated" people tend to use the vernacular and make many common errors, speaking in English of a lower register. (I don't know how many registers there are, or exactly how they're designated. It's probably just qualitative.)
I see it used as both a countable and uncountable noun. I believe your usage is a verb.
Sometimes "low[er] register" sounds like a compound adjective, if such there be.