My husband introduced me to this word when he found it in the petitions to the Supreme Court regarding firearms restrictions in the city of Chicago. (You can get PDFs of both petitions there - Chicago's lawyers decided to use the word "nugatory" and the reply seems to find that, uh, interesting. I'm not discussing the case here, just pointing out the word.)
I thought it had something to do with nougat, creamy, tasty nougat before I was given any context.
We both hoped it had something to do with The Nuge because the use involved guns. I was thinking along the lines of: "That action was so deeply stupid, trashy, and ignorant as to be nugatory." Alas, no.