I initially phrased this question in a comment to another question regarding what N1 and N2 mean, hoping it would be that sort of low-hanging fruit that is easily answerable and not worth a proper question in its own right. But then I starting searching and came up short.
This Yahoo Answers page has lots of guesses on it, from "nominal" to "Newell". (And we all now are reminded why nobody goes to that site for real answers.) Many other pages explain what N1 and N2 are, but not where the "N" part comes from.
I'm wondering if it might have its origin in German, as a lot of early developmental work on jet engines was done in Germany. Or was it simply an unused letter that fit?